pankajkulsange / JS-Interview-Preparation-Day3

What are promises and why do we need it?

• promise are use to handle async operations in JS
• easy to handle callback hell
• also use to handle the errors

1. Pending
2. Resolve (then method)
3. Reject (catch method)

• initial stage of any promise is always pending

Q. We have three promise, and we want to combine all then catch method by using promise.all

Q. promise chaining?

• technique to chain mulitple asynchronous operation in a specific order by chaining together using promise

Q. DOM - document object model

• mechanism by which we can change the document structure
• structure html document elements on the web
• we can use document object and it's method for change the document structure such as toggle the button and etc
• getElementById
• getElementsByClassName
• getElementsByTagName
• querySelector
• querySelectorAll
• addEventListener(event, callbackfunction)

Q. Closure

• combination of two or more nested functions
• where the inner function access the variable for the parent function
• in it's lexical environment called closure

Q. How many operators in JS

1. Arithmatic +, -, *, **, /, %, ++, --
2. Assingment =, +=, -=, *=, /=, %=
3. Bitwise bitwise or | , bitwise and &, bitwise not ~, left shift <<, right shift >>
4. Comparision ==, ===, <=, >=, !=, !==
5. Logical operator &&, ||, !
6. Ternary (condition) : true block code ? false block code;
7. typeof - return datatype

Q. Object

• non primitive datatype that stores key value pairs seperated by colon
• key also called Property

// method 2 - using new keyword let obj = new Object(); obj.age = 23; obj['name'] = 'Jane'