JavaScript

Question 1

What is the default prototype of a function?

Answer 1

an object with the only property constructor that points back to the function itself.

Question 2

What is a prototype in JavaScript?

Answer 2

Prototypes are the mechanism by which JavaScript objects inherit features from one another

Question 3

What is a class in JavaScript?

Answer 3

Syntactic sugar for JavaScript’s existing prototype-based inheritance

Question 4

What is the final link in any JavaScript Object’s prototypal chain?

Answer 4

technically, null, but `Object’ sits at the top of any JavaScript Object’s prototypal chain.

Question 5

What are JavaScripts 5 Primitive Data Types?

Answer 5

Number, String, Boolean, Undefined, Null

Question 6

What is NaN in JavaScript?

Answer 6

Stands for ‘Not a Number’ - it is the result of any illegal numerical operations.

Question 7

What are Falsy values in JavaScript?

Answer 7

0, “”, null, false, undefined, NaN

Question 8

What is a Primitive Data Type?

Answer 8

Data that is not an object and cannot have methods.

Question 9

What are the 5 different ways to declare variables or constants in JavaScript?

Answer 9

var, let, const, window, and global.

Question 10

What is the difference between var, let and const?

Answer 10

[var] - is function scoped, meaning that it will be hoisted to the top of the function in which it is defined.
[let] - is block scoped. meaning that it will be only be hoisted to top of the block in which it is declared.
[const] - used to declare constants. They cannot be redeclared, but they can be reassigned.

Question 11

What are the three different ways to declare a function in JavaScript?

Answer 11

1. Function-Style:
function functionName(arg1, arg2, arg3, argN) {
  // code block...
}
2. Expression-Style:
const functionName = function (arg1, arg2, arg3, argN) {
  // code block...
};
3. Fat-Arrow Style:
const functionName = (arg1, arg2, arg3, argN) => {
  // code block...
};

Question 12

What is a callback in JavaSrcipt?

Answer 12

A function that is passed to another function as an argument.

Question 13

What is a closure in JavaScript?

Answer 13

an inner function that has access to the outer (enclosing) function’s variables—scope chain.

Question 14

What is the difference between a pure and impure function?

Answer 14

A pure function is a function which given the same input, will always return the same value and it produces no side effects. An impure function is a function that mutates variables/state/data outside of it’s lexical scope.

Question 15

What is the new keyword?

Answer 15

The new keyword invokes a function in a special way. Functions invoked using the new keyword are called constructor functions.

1. Creates a new object.
2. Sets the object’s prototype to be the prototype of the constructor function.
3. Executes the constructor function with this as the newly created object.
4. Returns the created object. If the constructor returns an object, this object is returned.

Question 16

What are the main differences between ES5 and ES6 JavaScript?

Answer 16

1. Block Scoping
2. Lexical “this” (via Arrow Functions)
3. Dealing With “arguments” - in ES5 arguments were like an array, but not an array. In ES6, by using …args, args is an array.
4. Classes - JS doesn’t support the “Classes” and just simulates it via “prototypes”, it’s syntax has been very confusing for both existing JS developers and new comers who wants to use it in a traditional OO fashion.
5. Strict Mode - Strict Mode(“use strict”) helps identify common issues (or “bad” parts) and also helps with “securing” JavaScript.