React-Neagoie
React Components
Databases
JSON Placeholder Data
JavaScript Notes
Map
const myArray = [1,2,3,4]
myArray.map( el => el + 1)
// returns [2,3,4,5]
myArray.map( el => "c")
// returns ["c", "c", "c", "c"]
myArray.map(() => 1 )
// returns [1,1,1,1]
Promises
const myPromise = new Promise((resolve, reject) => {
if (true) {
setTimeout(() => {
resolve('I have succeeded');
}, 1000);
} else {
reject('I have failed!');
}
// 1000 is number of milliseconds, or 1 second in this case
myPromise.then(value => console.log(value));
})
Filter
Filter is similar to map in that it takes a function that iterates over each element in an array.
const myArray = [1,3,5,7,9]
myArray.filter( el => el > 5)
// returns [5,7,9]
Includes
We can use "includes" with one argument to determine whether a value exists in an array.
const myArray = [1,3,5,7,9]
myArray.includes(3)
// returns true
The second argument in includes() can be an index we want to start searching.
const myArray = [1,3,5,7,9]
myArray.includes(3,2)
// returns false
// it's asking if "3" exists after index=2 (which is 5); it does not, hence false
However, this method only works with primitive data types. For other data types (arrays, objects, etc), this doesn't work because we are pointing to a specific location in the memory. For instance, this returns false:
const myArray = [ {id: 1} ]
myArray.includes({id:1)
// returns false
In order to make it work for an object, we'd need something like this:
const o1 = {id: 1}
const o2 = {id: 2}
myArray = [o1, o2]
myArray.includes(o1)
// returns true