lulu27753/30-seconds-of-code

Array

Array difference(数组之间的区别)

Create a Set from b, then use Array.filter() on a to only keep values not contained in b. 从b创建一个Set，然后在a上使用Array.filter（），只保留b中不包含的值。

const difference = (a, b) => { const s = new Set(b); return a.filter(x => !s.has(x)); };
// difference([1,2,3], [1,2,4]) -> [3]

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Array intersection(两点之间的距离)

Create a Set from b, then use Array.filter() on a to only keep values contained in b. 使用Math.hypot（）计算两点之间的欧几里德距离。

const intersection = (a, b) => { const s = new Set(b); return a.filter(x => s.has(x)); };
// intersection([1,2,3], [4,3,2]) -> [2,3]

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Array pull (mutates array)

Use Array.filter() and Array.includes() to pull out the values that are not needed. Use Array.length = 0 to mutate the passed in array by resetting it's length to zero and Array.push() to re-populate it with only the pulled values.

const pull = (arr, ...args) => {
  let pulled = arr.filter((v, i) => args.includes(v));
  arr.length = 0; pulled.forEach(v => arr.push(v));
};
// let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
// pull(myArray, 'a', 'c');
// console.log(myArray) -> [ 'b', 'b' ]

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Array remove

Use Array.filter() to find array elements that return truthy values and Array.reduce() to remove elements using Array.splice(). The func is invoked with three arguments (value, index, array).

const remove = (arr, func) =>
  Array.isArray(arr) ? arr.filter(func).reduce((acc, val) => {
    arr.splice(arr.indexOf(val), 1); return acc.concat(val);
    }, [])
  : [];
// remove([1, 2, 3, 4], n => n % 2 == 0) -> [2, 4]

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Array sample

Use Math.random() to generate a random number, multiply it with length and round it of to the nearest whole number using Math.floor(). This method also works with strings.

const sample = arr => arr[Math.floor(Math.random() * arr.length)];
// sample([3, 7, 9, 11]) -> 9

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Array symmetric difference

Create a Set from each array, then use Array.filter() on each of them to only keep values not contained in the other.

const symmetricDifference = (a, b) => {
  const sA = new Set(a), sB = new Set(b);
  return [...a.filter(x => !sB.has(x)), ...b.filter(x => !sA.has(x))];
}
// symmetricDifference([1,2,3], [1,2,4]) -> [3,4]

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Array union

Create a Set with all values of a and b and convert to an array.

const union = (a, b) => Array.from(new Set([...a, ...b]));
// union([1,2,3], [4,3,2]) -> [1,2,3,4]

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Array without

Use Array.filter() to create an array excluding(using !Array.includes()) all given values.

const without = (arr, ...args) => arr.filter(v => !args.includes(v));
// without([2, 1, 2, 3], 1, 2) -> [3]
// without([2, 1, 2, 3, 4, 5, 5, 5, 3, 2, 7, 7], 3, 1, 5, 2) -> [ 4, 7, 7 ]

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Array zip

Use Math.max.apply() to get the longest array in the arguments. Creates an array with that length as return value and use Array.from() with a map-function to create an array of grouped elements. If lengths of the argument-arrays vary, undefined is used where no value could be found.

const zip = (...arrays) => {
  const maxLength = Math.max(...arrays.map(x => x.length));
  return Array.from({length: maxLength}).map((_, i) => {
   return Array.from({length: arrays.length}, (_, k) => arrays[k][i]);
  })
}
//zip(['a', 'b'], [1, 2], [true, false]); -> [['a', 1, true], ['b', 2, false]]
//zip(['a'], [1, 2], [true, false]); -> [['a', 1, true], [undefined, 2, false]]

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Average of array of numbers(数组平均数)

Use Array.reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array. 使用reduce（）将每个值添加到累加器，初始值为0，总和除以数组长度。

const average = arr => arr.reduce((acc, val) => acc + val, 0) / arr.length;
// average([1,2,3]) -> 2

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Chunk array

Use Array.from() to create a new array, that fits the number of chunks that will be produced. Use Array.slice() to map each element of the new array to a chunk the length of size. If the original array can't be split evenly, the final chunk will contain the remaining elements.

const chunk = (arr, size) =>
  Array.from({length: Math.ceil(arr.length / size)}, (v, i) => arr.slice(i * size, i * size + size));
// chunk([1,2,3,4,5], 2) -> [[1,2],[3,4],[5]]

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Compact

Use Array.filter() to filter out falsey values (false, null, 0, "", undefined, and NaN).

const compact = (arr) => arr.filter(Boolean);
// compact([0, 1, false, 2, '', 3, 'a', 'e'*23, NaN, 's', 34]) -> [ 1, 2, 3, 'a', 's', 34 ]

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Count occurrences of a value in array(计数数组中值的出现次数)

Use Array.reduce() to increment a counter each time you encounter the specific value inside the array. 每次遇到数组中的特定值时，使用reduce（）来递增计数器。

const countOccurrences = (arr, value) => arr.reduce((a, v) => v === value ? a + 1 : a + 0, 0);
// countOccurrences([1,1,2,1,2,3], 1) -> 3

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Deep flatten array

Use recursion. Use Array.concat() with an empty array ([]) and the spread operator (...) to flatten an array. Recursively flatten each element that is an array. 使用递归，使用reduce（）来获取所有不是数组的元素，flatten每个元素都是数组。

const deepFlatten = arr => [].concat(...arr.map(v => Array.isArray(v) ? deepFlatten(v) : v));
// deepFlatten([1,[2],[[3],4],5]) -> [1,2,3,4,5]

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Drop elements in array

Loop through the array, using Array.shift() to drop the first element of the array until the returned value from the function is true. Returns the remaining elements.

const dropElements = (arr, func) => {
  while (arr.length > 0 && !func(arr[0])) arr.shift();
  return arr;
};
// dropElements([1, 2, 3, 4], n => n >= 3) -> [3,4]

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Filter out non-unique values in an array(过滤数组中的非唯一值)

Use Array.filter() for an array containing only the unique values. 将Array.filter（）用于仅包含唯一值的数组。

const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
// filterNonUnique([1,2,2,3,4,4,5]) -> [1,3,5]

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Flatten array up to depth

Use recursion, decrementing depth by 1 for each level of depth. Use Array.reduce() and Array.concat() to merge elements or arrays. Base case, for depth equal to 1 stops recursion. Omit the second element, depth to flatten only to a depth of 1 (single flatten).

const flattenDepth = (arr, depth = 1) =>
  depth != 1 ? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flattenDepth(v, depth - 1) : v), [])
  : arr.reduce((a, v) => a.concat(v), []);
// flattenDepth([1,[2],[[[3],4],5]], 2) -> [1,2,[3],4,5]

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Flatten array(Flatten数组)

Use Array.reduce() to get all elements inside the array and concat() to flatten them. 使用reduce（）来获取数组中的所有元素，并使用concat（）来使它们flatten。

const flatten = arr => arr.reduce((a, v) => a.concat(v), []);
// flatten([1,[2],3,4]) -> [1,2,3,4]

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Get max value from array(从数组中获取最大值)

Use Math.max() combined with the spread operator (...) to get the maximum value in the array. 使用Math.max（）与spread运算符（...）结合得到数组中的最大值。

const arrayMax = arr => Math.max(...arr);
// arrayMax([10, 1, 5]) -> 10

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Get min value from array(从数组中获取最小值)

Use Math.min() combined with the spread operator (...) to get the minimum value in the array. 使用Math.min（）与spread运算符（...）结合得到数组中的最小值。

const arrayMin = arr => Math.min(...arr);
// arrayMin([10, 1, 5]) -> 1

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Group by

Use Array.map() to map the values of an array to a function or property name. Use Array.reduce() to create an object, where the keys are produced from the mapped results.

const groupBy = (arr, func) =>
  arr.map(typeof func === 'function' ? func : val => val[func])
    .reduce((acc, val, i) => { acc[val] = (acc[val] || []).concat(arr[i]); return acc; }, {});
// groupBy([6.1, 4.2, 6.3], Math.floor) -> {4: [4.2], 6: [6.1, 6.3]}
// groupBy(['one', 'two', 'three'], 'length') -> {3: ['one', 'two'], 5: ['three']}

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Head of list

Use arr[0] to return the first element of the passed array. 返回ARR[0]

const head = arr => arr[0];
// head([1,2,3]) -> 1

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Initial of list(list初始化)

Use arr.slice(0,-1)to return all but the last element of the array. 返回arr.slice（0，-1）

const initial = arr => arr.slice(0, -1);
// initial([1,2,3]) -> [1,2]

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Initialize array with range(用range初始化数组)

Use Array(end-start) to create an array of the desired length, Array.map() to fill with the desired values in a range. You can omit start to use a default value of 0. 使用Array（end-start）创建所需长度的数组，使用map（）来填充范围中的所需值，可以省略start使用默认值0。

const initializeArrayRange = (end, start = 0) =>
  Array.from({ length: end - start }).map((v, i) => i + start);
// initializeArrayRange(5) -> [0,1,2,3,4]

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Initialize array with values(用值初始化数组)

Use Array(n) to create an array of the desired length, fill(v) to fill it with the desired values. You can omit value to use a default value of 0. 使用Array（n）创建所需长度的数组，fill(v)以填充所需的值，可以忽略value使用默认值0。

const initializeArray = (n, value = 0) => Array(n).fill(value);
// initializeArray(5, 2) -> [2,2,2,2,2]

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Last of list(列表的最后)

Use arr.length - 1 to compute index of the last element of the given array and returning it. 返回arr.slice（-1）[0]

const last = arr => arr[arr.length - 1];
// last([1,2,3]) -> 3

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Median of array of numbers

Find the middle of the array, use Array.sort() to sort the values. Return the number at the midpoint if length is odd, otherwise the average of the two middle numbers.

const median = arr => {
  const mid = Math.floor(arr.length / 2), nums = arr.sort((a, b) => a - b);
  return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
};
// median([5,6,50,1,-5]) -> 5
// median([0,10,-2,7]) -> 3.5

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Nth element of array

Use Array.slice() to get an array containing the nth element at the first place. If the index is out of bounds, return []. Omit the second argument, n, to get the first element of the array.

const nth = (arr, n=0) => (n>0? arr.slice(n,n+1) : arr.slice(n))[0];
// nth(['a','b','c'],1) -> 'b'
// nth(['a','b','b']-2) -> 'a'

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Pick

Use Array.reduce() to convert the filtered/picked keys back to a object with the corresponding key:value pair if the key exist in the obj.

const pick = (obj, arr) =>
  arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});
// pick({ 'a': 1, 'b': '2', 'c': 3 }, ['a', 'c']) -> { 'a': 1, 'c': 3 }
// pick(object, ['a', 'c'])['a'] -> 1

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Shuffle array(随机化数组的顺序)

Use Array.sort() to reorder elements, using Math.random() in the comparator. 使用sort（）重新排序元素，利用Math.random（）来随机排序。

const shuffle = arr => arr.sort(() => Math.random() - 0.5);
// shuffle([1,2,3]) -> [2,3,1]

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Similarity between arrays(数组之间的相似性)

Use filter() to remove values that are not part of values, determined using includes(). 使用filter（）移除不是values的一部分值，使用includes（）确定。

const similarity = (arr, values) => arr.filter(v => values.includes(v));
// similarity([1,2,3], [1,2,4]) -> [1,2]

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Sum of array of numbers(数组总和)

Use Array.reduce() to add each value to an accumulator, initialized with a value of 0. 使用reduce（）将每个值添加到累加器，初始化值为0。

const sum = arr => arr.reduce((acc, val) => acc + val, 0);
// sum([1,2,3,4]) -> 10

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Tail of list(列表的tail)

Return arr.slice(1) if the array's length is more than 1, otherwise return the whole array. 返回arr.slice（1）

const tail = arr => arr.length > 1 ? arr.slice(1) : arr;
// tail([1,2,3]) -> [2,3]
// tail([1]) -> [1]

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Take every nth element

Use Array.filter() to create a new array that contains every nth element of a given array.

const everynth = (arr, nth) => arr.filter((e, i) => i % nth === 0);
// everynth([1,2,3,4,5,6], 2) -> [ 1, 3, 5 ]

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Take right

Use Array.slice() to create a slice of the array with n elements taken from the end.

const takeRight = (arr, n = 1) => arr.slice(arr.length - n, arr.length);
// takeRight([1, 2, 3], 2) -> [ 2, 3 ]
// takeRight([1, 2, 3]) -> [3]

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Take

Use Array.slice() to create a slice of the array with n elements taken from the beginning.

const take = (arr, n = 1) => arr.slice(0, n);
// take([1, 2, 3], 5) -> [1, 2, 3]
// take([1, 2, 3], 0) -> []

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Unique values of array(数组唯一值)

Use ES6 Set and the ...rest operator to discard all duplicated values. 使用ES6 Set和... rest操作符去掉所有重复值。

const unique = arr => [...new Set(arr)];
// unique([1,2,2,3,4,4,5]) -> [1,2,3,4,5]

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Browser

Bottom visible

Use scrollY, scrollHeight and clientHeight to determine if the bottom of the page is visible.

const bottomVisible = () =>
  document.documentElement.clientHeight + window.scrollY >= document.documentElement.scrollHeight || document.documentElement.clientHeight;
// bottomVisible() -> true

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Current URL(当前URL)

Use window.location.href to get current URL. 使用window.location.href来获取当前URL。

const currentUrl = () => window.location.href;
// currentUrl() -> 'https://google.com'

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Element is visible in viewport

Use Element.getBoundingClientRect() and the window.inner(Width|Height) values to determine if a given element is visible in the viewport. Omit the second argument to determine if the element is entirely visible, or specify true to determine if it is partially visible.

const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
  const { top, left, bottom, right } = el.getBoundingClientRect();
  return partiallyVisible
    ? ((top > 0 && top < innerHeight) || (bottom > 0 && bottom < innerHeight)) &&
      ((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
    : top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
};
// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
// elementIsVisibleInViewport(el) -> false (not fully visible)
// elementIsVisibleInViewport(el, true) -> true (partially visible)

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Get scroll position(获取滚动位置)

Use pageXOffset and pageYOffset if they are defined, otherwise scrollLeft and scrollTop. You can omit el to use a default value of window. 如果已定义，请使用pageXOffset和pageYOffset，否则使用scrollLeft和scrollTop，可以省略el来使用window的默认值。

const getScrollPos = (el = window) =>
  ({x: (el.pageXOffset !== undefined) ? el.pageXOffset : el.scrollLeft,
    y: (el.pageYOffset !== undefined) ? el.pageYOffset : el.scrollTop});
// getScrollPos() -> {x: 0, y: 200}

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Redirect to URL(重定向到URL)

Use window.location.href or window.location.replace() to redirect to url. Pass a second argument to simulate a link click (true - default) or an HTTP redirect (false). 使用window.location.href或window.location.replace（）重定向到url。传递第二个参数来模拟链接点击（true - default）或HTTP重定向（false）。

const redirect = (url, asLink = true) =>
  asLink ? window.location.href = url : window.location.replace(url);
// redirect('https://google.com')

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Scroll to top(滚动到顶部)

Get distance from top using document.documentElement.scrollTop or document.body.scrollTop. Scroll by a fraction of the distance from top. Use window.requestAnimationFrame() to animate the scrolling. 使用document.documentElement.scrollTop或document.body.scrollTop获取到顶部的距离。从顶部滚动一小部分距离。

使用window.requestAnimationFrame（）来滚动。

const scrollToTop = () => {
  const c = document.documentElement.scrollTop || document.body.scrollTop;
  if (c > 0) {
    window.requestAnimationFrame(scrollToTop);
    window.scrollTo(0, c - c / 8);
  }
};
// scrollToTop()

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Date

Convert to English date

Use Date.toISOString(), split('T') and replace() to convert a date from American format to English format. Throws an error if the passed time cannot be converted to a date.

const toEnglishDate  = (time) =>
  {try{return new Date(time).toISOString().split('T')[0].replace(/-/g, '/')}catch(e){return}};
// toEnglishDate('09/21/2010') -> '21/09/2010'

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Get days difference between dates

Calculate the difference (in days) between to Date objects.

const getDaysDiffBetweenDates = (dateInitial, dateFinal) => (dateFinal - dateInitial) / (1000 * 3600 * 24);
// getDaysDiffBetweenDates(new Date("2017-12-13"), new Date("2017-12-22")) -> 9

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JSON to date

Use Date(), to convert dates in JSON format to readable format (dd/mm/yyyy).

const jsonToDate = arr => {
  const dt = new Date(parseInt(arr.toString().substr(6)));
  return `${ dt.getDate() }/${ dt.getMonth() + 1 }/${ dt.getFullYear() }`
};
// jsonToDate(/Date(1489525200000)/) -> "14/3/2017"

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Function

Chain asynchronous functions

Loop through an array of functions containing asynchronous events, calling next when each asynchronous event has completed.

const chainAsync = fns => { let curr = 0; const next = () => fns[curr++](next); next(); };
/*
chainAsync([
  next => { console.log('0 seconds'); setTimeout(next, 1000); },
  next => { console.log('1 second');  setTimeout(next, 1000); },
  next => { console.log('2 seconds'); }
])
*/

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Compose functions

Use Array.reduce() to perform right-to-left function composition. The last (rightmost) function can accept one or more arguments; the remaining functions must be unary.

const compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));
/*
const add5 = x => x + 5
const multiply = (x, y) => x * y
const multiplyAndAdd5 = compose(add5, multiply)
multiplyAndAdd5(5, 2) -> 15
*/

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Curry

Use recursion. If the number of provided arguments (args) is sufficient, call the passed function f. Otherwise return a curried function f that expects the rest of the arguments. If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g. Math.min()), you can optionally pass the number of arguments to the second parameter arity. 使用递归。如果提供的参数（args）数量足够，则调用传递函数f，否则返回一个curried函数f。

const curry = (fn, arity = fn.length, ...args) =>
  arity <= args.length
    ? fn(...args)
    : curry.bind(null, fn, arity, ...args);
// curry(Math.pow)(2)(10) -> 1024
// curry(Math.min, 3)(10)(50)(2) -> 2

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Log function name

Use console.debug() and the name property of the passed method to log the method's name to the debug channel of the console.

const functionName = fn => (console.debug(fn.name), fn);
// functionName(Math.max) -> max (logged in debug channel of console)

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Pipe functions(管道)

Use Array.reduce() with the spread operator (...) to perform left-to-right function composition. The first (leftmost) function can accept one or more arguments; the remaining functions must be unary. 使用Array.reduce（）通过函数传递值。

const pipe = (...fns) => fns.reduce((f, g) => (...args) => g(f(...args)));
/*
const add5 = x => x + 5
const multiply = (x, y) => x * y
const multiplyAndAdd5 = pipe(multiply, add5)
multiplyAndAdd5(5, 2) -> 15
*/

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Promisify

Use currying to return a function returning a Promise that calls the original function. Use the ...rest operator to pass in all the parameters.

In Node 8+, you can use util.promisify

const promisify = func =>
  (...args) =>
    new Promise((resolve, reject) =>
      func(...args, (err, result) =>
        err ? reject(err) : resolve(result))
    );
// const delay = promisify((d, cb) => setTimeout(cb, d))
// delay(2000).then(() => console.log('Hi!')) -> Promise resolves after 2s

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Run promises in series

Run an array of promises in series using Array.reduce() by creating a promise chain, where each promise returns the next promise when resolved.

const series = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());
// const delay = (d) => new Promise(r => setTimeout(r, d))
// series([() => delay(1000), () => delay(2000)]) -> executes each promise sequentially, taking a total of 3 seconds to complete

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Sleep

Delay executing part of an async function, by putting it to sleep, returning a Promise.

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
/*
async function sleepyWork() {
  console.log('I\'m going to sleep for 1 second.');
  await sleep(1000);
  console.log('I woke up after 1 second.');
}
*/

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Math

Collatz algorithm

If n is even, return n/2. Otherwise return 3n+1.

const collatz = n => (n % 2 == 0) ? (n / 2) : (3 * n + 1);
// collatz(8) --> 4
// collatz(5) --> 16

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Distance between two points

Use Math.hypot() to calculate the Euclidean distance between two points.

const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);
// distance(1,1, 2,3) -> 2.23606797749979

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Divisible by number(可以按数字整除)

Use the modulo operator (%) to check if the remainder is equal to 0. 使用模运算符（％）来检查余数是否等于0。

const isDivisible = (dividend, divisor) => dividend % divisor === 0;
// isDivisible(6,3) -> true

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Even or odd number(偶数或奇数)

Checks whether a number is odd or even using the modulo (%) operator. Returns true if the number is even, false if the number is odd. 使用Math.abs（）将逻辑扩展为负数，使用模（％）运算符进行检查。如果数字是偶数，则返回true；如果数字是奇数，则返回false。

const isEven = num => num % 2 === 0;
// isEven(3) -> false

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Factorial(阶乘)

Use recursion. If n is less than or equal to 1, return 1. Otherwise, return the product of n and the factorial of n - 1. Throws an exception if n is a negative number. 使用递归。如果n小于或等于1，则返回1。否则返回n和n - 1的阶乘的乘积。

const factorial = n =>
  n < 0 ? (() => { throw new TypeError('Negative numbers are not allowed!') })()
  : n <= 1 ? 1 : n * factorial(n - 1);
// factorial(6) -> 720

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Fibonacci array generator(斐波那契数组生成器)

Create an empty array of the specific length, initializing the first two values (0 and 1). Use Array.reduce() to add values into the array, using the sum of the last two values, except for the first two. 创建一个特定长度的空数组，初始化前两个值（0和1）。使用Array.reduce（）向数组中添加值，后面的一个数等于前面两个数相加之和（前两个除外）。

const fibonacci = n =>
  Array(n).fill(0).reduce((acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i), []);
// fibonacci(5) -> [0,1,1,2,3]

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Greatest common divisor (GCD)(最大公约数)

Use recursion. Base case is when y equals 0. In this case, return x. Otherwise, return the GCD of y and the remainder of the division x/y. 使用递归。基本情况是当y等于0时。在这种情况下，返回x。否则，返回y的GCD和x / y的其余部分。

const gcd = (x, y) => !y ? x : gcd(y, x % y);
// gcd (8, 36) -> 4

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Hamming distance

Use XOR operator (^) to find the bit difference between the two numbers, convert to binary string using toString(2). Count and return the number of 1s in the string, using match(/1/g).

const hammingDistance = (num1, num2) =>
  ((num1 ^ num2).toString(2).match(/1/g) || '').length;
// hammingDistance(2,3) -> 1

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Least common multiple (LCM)

Use the greatest common divisor (GCD) formula and Math.abs() to determine the least common multiple. The GCD formula uses recursion.

const lcm = (x,y) => {
  const gcd = (x, y) => !y ? x : gcd(y, x % y);
  return Math.abs(x*y)/(gcd(x,y));
};
// lcm(12,7) -> 84

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Percentile

Use Array.reduce() to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.

const percentile = (arr, val) => 
  100 * arr.reduce((acc,v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0) / arr.length;
// percentile([1,2,3,4,5,6,7,8,9,10], 6) -> 55

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Powerset

Use Array.reduce() combined with Array.map() to iterate over elements and combine into an array containing all combinations. 使用reduce（）与map（）结合来遍历元素，并将其组合成包含所有组合的数组。

const powerset = arr =>
  arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
// powerset([1,2]) -> [[], [1], [2], [2,1]]

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Random integer in range(范围内的随机整数)

Use Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer. 使用Math.random（）生成一个随机数并将其映射到所需的范围，使用Math.floor（）使其成为一个整数。

const randomIntegerInRange = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;
// randomIntegerInRange(0, 5) -> 2

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Random number in range(范围内的随机数)

Use Math.random() to generate a random value, map it to the desired range using multiplication. 使用Math.random（）生成一个随机值，使用乘法将其映射到所需的范围。

const randomInRange = (min, max) => Math.random() * (max - min) + min;
// randomInRange(2,10) -> 6.0211363285087005

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Round number to n digits

Use Math.round() and template literals to round the number to the specified number of digits. Omit the second argument, decimals to round to an integer.

const round = (n, decimals=0) => Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);
// round(1.005, 2) -> 1.01

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Standard deviation

Use Array.reduce() to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then determine the standard deviation. You can omit the second argument to get the sample standard deviation or set it to true to get the population standard deviation.

const standardDeviation = (arr, usePopulation = false) => {
  const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
  return Math.sqrt(
    arr.reduce((acc, val) => acc.concat(Math.pow(val - mean, 2)), [])
       .reduce((acc, val) => acc + val, 0) / (arr.length - (usePopulation ? 0 : 1))
  );
};
// standardDeviation([10,2,38,23,38,23,21]) -> 13.284434142114991 (sample)
// standardDeviation([10,2,38,23,38,23,21], true) -> 12.29899614287479 (population)

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Media

Speech synthesis (experimental)

Use SpeechSynthesisUtterance.voice and window.speechSynthesis.getVoices() to convert a message to speech. Use window.speechSynthesis.speak() to play the message.

Learn more about the SpeechSynthesisUtterance interface of the Web Speech API.

const speak = message => {
  const msg = new SpeechSynthesisUtterance(message);
  msg.voice = window.speechSynthesis.getVoices()[0];
  window.speechSynthesis.speak(msg);
};
// speak('Hello, World') -> plays the message

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Node

Read file as array of lines

Use readFileSync function in fs node package to create a Buffer from a file. convert buffer to string using toString(encoding) function. creating an array from contents of file by spliting file content line by line(each \n).

const fs = require('fs');
const readFileToArray = filename => fs.readFileSync(filename).toString('UTF8').split('\n');
/*
contents of test.txt :
  line1
  line2
  line3
  ___________________________
let arr = readFileToArray('test.txt')
console.log(arr) // -> ['line1', 'line2', 'line3']
*/

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Write JSON to file

Use fs.writeFile(), template literals and JSON.stringify() to write a json object to a .json file.

const fs = require('fs');
const jsonToFile = (obj, filename) => fs.writeFile(`${filename}.json`, JSON.stringify(obj, null, 2))
// jsonToFile({test: "is passed"}, 'testJsonFile') -> writes the object to 'testJsonFile.json'

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Object

Clean JSON object

Use Object.keys() method to loop over given json object and deleting keys that are not included in given array. also if you give it a special key(childIndicator) it will search deeply inside it to apply function to inner objects too.

const cleanObj = (obj, keysToKeep = [], childIndicator) => {
  Object.keys(obj).forEach(key => {
    if (key === childIndicator) {
      cleanObj(obj[key], keysToKeep, childIndicator);
    } else if (!keysToKeep.includes(key)) {
      delete obj[key];
    }
  })
}
/*
  testObj = {a: 1, b: 2, children: {a: 1, b: 2}}
  cleanObj(testObj, ["a"],"children")
  console.log(testObj)// { a: 1, children : { a: 1}}
*/

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Object from key-value pairs(来自键值对的对象)

Use Array.reduce() to create and combine key-value pairs. 使用Array.reduce（）来创建和组合键值对。

const objectFromPairs = arr => arr.reduce((a, v) => (a[v[0]] = v[1], a), {});
// objectFromPairs([['a',1],['b',2]]) -> {a: 1, b: 2}

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Object to key-value pairs

Use Object.keys() and Array.map() to iterate over the object's keys and produce an array with key-value pairs.

const objectToPairs = obj => Object.keys(obj).map(k => [k, obj[k]]);
// objectToPairs({a: 1, b: 2}) -> [['a',1],['b',2]])

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Shallow clone object

Use Object.assign() and an empty object ({}) to create a shallow clone of the original.

const shallowClone = obj => Object.assign({}, obj);
/*
const a = { x: true, y: 1 };
const b = shallowClone(a);
a === b -> false
*/

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String

Anagrams of string (with duplicates)

Use recursion. For each letter in the given string, create all the partial anagrams for the rest of its letters. Use Array.map() to combine the letter with each partial anagram, then Array.reduce() to combine all anagrams in one array. Base cases are for string length equal to 2 or 1. 使用递归。对于给定字符串中的每个字母，为字母创建字谜。使用map（）将字母与每部分字谜组合，然后使用reduce（）将所有字谜组合到一个数组中，最基本情况是字符串长度等于2或1。

const anagrams = str => {
  if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
  return str.split('').reduce((acc, letter, i) =>
    acc.concat(anagrams(str.slice(0, i) + str.slice(i + 1)).map(val => letter + val)), []);
};
// anagrams('abc') -> ['abc','acb','bac','bca','cab','cba']

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Capitalize first letter of every word(大写每个单词的首字母)

Use replace() to match the first character of each word and toUpperCase() to capitalize it. 使用replace（）匹配每个单词的第一个字符，并使用toUpperCase（）来将其大写。

const capitalizeEveryWord = str => str.replace(/\b[a-z]/g, char => char.toUpperCase());
// capitalizeEveryWord('hello world!') -> 'Hello World!'

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Capitalize first letter(首字母大写)

Use destructuring and toUpperCase() to capitalize first letter, ...rest to get array of characters after first letter and then Array.join('') to make it a string again. Omit the lowerRest parameter to keep the rest of the string intact, or set it to true to convert to lower case. 使用slice（0,1）和toUpperCase（）大写第一个字母，slice（1）获取字符串的其余部分。省略lowerRest参数以保持字符串的其余部分不变，或将其设置为true以转换为小写。（注意：这和上一个示例不是同一件事情）

const capitalize = ([first,...rest], lowerRest = false) =>
  first.toUpperCase() + (lowerRest ? rest.join('').toLowerCase() : rest.join(''));
// capitalize('myName') -> 'MyName'
// capitalize('myName', true) -> 'Myname'

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Check for palindrome(检查回文)

Convert string toLowerCase() and use replace() to remove non-alphanumeric characters from it. Then, split('') into individual characters, reverse(), join('') and compare to the original, unreversed string, after converting it tolowerCase(). 将字符串转换为toLowerCase（），并使用replace（）从中删除非字母的字符。然后，将其转换为tolowerCase（），将（''）拆分为单独字符，reverse（），join（''），与原始的非反转字符串进行比较，然后将其转换为tolowerCase（）。

const palindrome = str => {
  const s = str.toLowerCase().replace(/[\W_]/g,'');
  return s === s.split('').reverse().join('');
}
// palindrome('taco cat') -> true

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Convert string from camelcase

Use replace() to remove underscores, hyphens and spaces and convert words to camelcase. Omit the scond argument to use a default separator of '_'.

const fromCamelCase = (str, separator = '_') => 
  str.replace(/([a-z\d])([A-Z])/g, '$1' + separator + '$2')
    .replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + separator + '$2').toLowerCase();
// fromCamelCase('someDatabaseFieldName', ' ') -> 'some database field name'
// fromCamelCase('someLabelThatNeedsToBeCamelized', '-') -> 'some-label-that-needs-to-be-camelized'
// fromCamelCase('someJavascriptProperty', '_') -> 'some_javascript_property'

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Convert string to camelcase

Use replace() to remove underscores, hyphens and spaces and convert words to camelcase.

const toCamelCase = str => 
  str.replace(/^([A-Z])|[\s-_]+(\w)/g, (match, p1, p2, offset) =>  p2 ? p2.toUpperCase() : p1.toLowerCase());
// toCamelCase("some_database_field_name") -> 'someDatabaseFieldName'
// toCamelCase("Some label that needs to be camelized") -> 'someLabelThatNeedsToBeCamelized'
// toCamelCase("some-javascript-property") -> 'someJavascriptProperty'
// toCamelCase("some-mixed_string with spaces_underscores-and-hyphens") -> 'someMixedStringWithSpacesUnderscoresAndHyphens'

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Reverse a string(反转一个字符串)

Use array destructuring and Array.reverse() to reverse the order of the characters in the string. Combine characters to get a string using join(''). 使用数组解构和Array.reverse（）来颠倒字符串中的字符顺序。合并字符以使用join('')获取字符串。

const reverseString = str => [...str].reverse().join('');
// reverseString('foobar') -> 'raboof'

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Sort characters in string (alphabetical)(按字符串排序（按字母顺序排列）)

Split the string using split(''), Array.sort() utilizing localeCompare(), recombine using join(''). 使用split（''）分割字符串，sort（）使用localeCompare（），使用join（''）重新组合。

const sortCharactersInString = str =>
  str.split('').sort((a, b) => a.localeCompare(b)).join('');
// sortCharactersInString('cabbage') -> 'aabbceg'

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Truncate a String

Determine if the string's length is greater than num. Return the string truncated to the desired length, with ... appended to the end or the original string.

const truncate = (str, num) =>
  str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;
// truncate('boomerang', 7) -> 'boom...'

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Utility

3-digit hexcode to 6-digit hexcode

Use Array.map(), split() and Array.join() to join the mapped array for converting a three-digit RGB notated hexadecimal color-code to the six-digit form. Array.slice() is used to remove # from string start since it's added once.

const convertHex = shortHex =>
  '#' + shortHex.slice(shortHex.startsWith('#') ? 1 : 0).split().map(x => x+x).join()
// convertHex('#03f') -> '#0033ff'
// convertHex('05a') -> '#0055aa'

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Escape regular expression(转义正则表达式)

Use replace() to escape special characters. 使用replace（）来转义特殊字符。

const escapeRegExp = str => str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
// escapeRegExp('(test)') -> \\(test\\)

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Get native type of value

Returns lower-cased constructor name of value, "undefined" or "null" if value is undefined or null

const getType = v =>
  v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name.toLowerCase();
// getType(new Set([1,2,3])) -> "set"

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Hexcode to RGB

Use bitwise right-shift operator and mask bits with & (and) operator to convert a hexadecimal color code (prefixed with #) to a string with the RGB values.

const hexToRgb = hex => {
  const h = parseInt(hex.slice(1), 16);
  return `rgb(${h >> 16}, ${(h & 0x00ff00) >> 8}, ${h & 0x0000ff})`;
}
// hexToRgb('#27ae60') -> 'rgb(39, 174, 96)'

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Is array

Use Array.isArray() to check if a value is classified as an array.

const isArray = val => !!val && Array.isArray(val);
// isArray(null) -> false
// isArray([1]) -> true

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Is boolean

Use typeof to check if a value is classified as a boolean primitive.

const isBoolean = val => typeof val === 'boolean';
// isBoolean(null) -> false
// isBoolean(false) -> true

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Is function

Use typeof to check if a value is classified as a function primitive.

const isFunction = val => val && typeof val === 'function';
// isFunction('x') -> false
// isFunction(x => x) -> true

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Is number

Use typeof to check if a value is classified as a number primitive.

const isNumber = val => typeof val === 'number';
// isNumber('1') -> false
// isNumber(1) -> true

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Is string

Use typeof to check if a value is classified as a string primitive.

const isString = val => typeof val === 'string';
// isString(10) -> false
// isString('10') -> true

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Is symbol

Use typeof to check if a value is classified as a symbol primitive.

const isSymbol = val => typeof val === 'symbol';
// isSymbol('x') -> false
// isSymbol(Symbol('x')) -> true

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Measure time taken by function(测试功能所花费的时间)

Use console.time() and console.timeEnd() to measure the difference between the start and end times to determine how long the callback took to execute. 使用performance.now（）获取函数的开始和结束时间，console.log（）所花费的时间。第一个参数是函数名，随后的参数传递给函数。

const timeTaken = callback => {
  console.time('timeTaken');
  const r = callback();
  console.timeEnd('timeTaken');
  return r;
};
// timeTaken(() => Math.pow(2, 10)) -> 1024
// (logged): timeTaken: 0.02099609375ms

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Number to array of digits

Convert the number to a string, using spread operators in ES6([...string]) build an array. Use Array.map() and parseInt() to transform each value to an integer.

const digitize = n => [...''+n].map(i => parseInt(i));
// digitize(2334) -> [2, 3, 3, 4]

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Ordinal suffix of number

Use the modulo operator (%) to find values of single and tens digits. Find which ordinal pattern digits match. If digit is found in teens pattern, use teens ordinal.

const toOrdinalSuffix = num => {
  const int = parseInt(num), digits = [(int % 10), (int % 100)],
    ordinals = ['st', 'nd', 'rd', 'th'], oPattern = [1, 2, 3, 4],
    tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
  return oPattern.includes(digits[0]) && !tPattern.includes(digits[1]) ? int + ordinals[digits[0] - 1] : int + ordinals[3];
};
// toOrdinalSuffix("123") -> "123rd"

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RGB to hexadecimal(RGB到十六进制)

Convert given RGB parameters to hexadecimal string using bitwise left-shift operator (<<) and toString(16), then padStart(6,'0') to get a 6-digit hexadecimal value. 使用按位左移运算符（<<）和toString（16），然后padStart（6，“0”）将给定的RGB参数转换为十六进制字符串以获得6位十六进制值。

const rgbToHex = (r, g, b) => ((r << 16) + (g << 8) + b).toString(16).padStart(6, '0');
// rgbToHex(255, 165, 1) -> 'ffa501'

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URL parameters(URL参数)

Use match() with an appropriate regular expression to get all key-value pairs, Array.reduce() to map and combine them into a single object. Pass location.search as the argument to apply to the current url. 使用match() 与适当的正则表达式来获得所有键值对，适当的map() 。使用Object.assign（）和spread运算符（...）将所有键值对组合到一个对象中，将location.search作为参数传递给当前url。

const getUrlParameters = url =>
  url.match(/([^?=&]+)(=([^&]*))/g).reduce(
    (a, v) => (a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1), a), {}
  );
// getUrlParameters('http://url.com/page?name=Adam&surname=Smith') -> {name: 'Adam', surname: 'Smith'}

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UUID generator(UUID生成器)

Use crypto API to generate a UUID, compliant with RFC4122 version 4. 使用crypto API生成符合RFC4122版本4的UUID。

const uuid = () =>
  ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
    (c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> c / 4).toString(16)
  );
// uuid() -> '7982fcfe-5721-4632-bede-6000885be57d'

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Validate email

Use a regular expression to check if the email is valid. Returns true if email is valid, false if not.

const validateEmail = str =>
  /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/.test(str);
// validateEmail(mymail@gmail.com) -> true

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Validate number(验证数字)

Use !isNaN in combination with parseFloat() to check if the argument is a number. Use isFinite() to check if the number is finite. Use Number() to check if the coercion holds. 使用！isNaN和parseFloat（）来检查参数是否是一个数字，使用isFinite（）来检查数字是否是有限的。

const validateNumber = n => !isNaN(parseFloat(n)) && isFinite(n) && Number(n) == n;
// validateNumber('10') -> true

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Credits

Icons made by Smashicons from www.flaticon.com is licensed by CC 3.0 BY.

Table of Contents

Array

Browser

Date

Function

Math

Media

Node

Object

String

Utility

Array

Array difference(数组之间的区别)

Array intersection(两点之间的距离)

Array pull (mutates array)

Array remove

Array sample

Array symmetric difference

Array union

Array without

Array zip

Average of array of numbers(数组平均数)

Chunk array

Compact

Count occurrences of a value in array(计数数组中值的出现次数)

Deep flatten array

Drop elements in array

Filter out non-unique values in an array(过滤数组中的非唯一值)

Flatten array up to depth

Flatten array(Flatten数组)

Get max value from array(从数组中获取最大值)

Get min value from array(从数组中获取最小值)

Group by

Head of list

Initial of list(list初始化)

Initialize array with range(用range初始化数组)

Initialize array with values(用值初始化数组)

Last of list(列表的最后)

Median of array of numbers

Nth element of array

Pick

Shuffle array(随机化数组的顺序)

Similarity between arrays(数组之间的相似性)

Sum of array of numbers(数组总和)

Tail of list(列表的tail)

Take every nth element

Take right

Take

Unique values of array(数组唯一值)

Browser

Bottom visible

Current URL(当前URL)

Element is visible in viewport

Get scroll position(获取滚动位置)

Redirect to URL(重定向到URL)

Scroll to top(滚动到顶部)

Date

Convert to English date

Get days difference between dates

JSON to date

Function

Chain asynchronous functions

Compose functions

Curry

Log function name

Pipe functions(管道)

Promisify

Run promises in series

Sleep

Math

Collatz algorithm

Distance between two points

Divisible by number(可以按数字整除)

Even or odd number(偶数或奇数)

Factorial(阶乘)

Fibonacci array generator(斐波那契数组生成器)

Greatest common divisor (GCD)(最大公约数)

Hamming distance

Least common multiple (LCM)

Percentile