Javascript fundamentals

Primitive data types

Immutable values stored by value. Includes: string, number, boolean, null, undefined, symbol, bigint.

Reference data types

Stored by reference (memory address). Includes: objects, arrays, functions.

Difference between primitive and reference types

Primitives are copied by value, references are copied by memory reference.

var

Function-scoped, hoisted with undefined, can be redeclared and reassigned.

let

Block-scoped, hoisted but in Temporal Dead Zone, can be reassigned but not redeclared.

const

Block-scoped, must be initialized, cannot be reassigned.

Can const objects be mutated?

Yes, object properties can change, but the variable binding cannot.

Hoisting

JavaScript moves declarations of variables, functions, and classes to the top of their scope before execution.

Are variables hoisted?

Yes, but only declarations, not initializations.

Why block scope is important

Prevents accidental variable access and bugs.

Closure

A function that remembers variables from its outer scope even after the outer function has finished executing.

Common closure use cases

Data encapsulation, private variables, callbacks, event handlers.

Example of closure

function outer() {
let x = 10;
return function inner() {
return x;
}
}

this keyword

Refers to the object that is executing the current function

this in global scope

In browser: window; in strict mode: undefined.

call()

Invokes a function immediately with a specified this and arguments passed individually.

apply()

Invokes a function immediately with a specified this and arguments as an array.

bind()

Returns a new function with permanently bound this.

Difference between call and apply

call uses arguments list, apply uses arguments array.

When to use bind

When passing methods as callbacks and you want fixed this.

What is JavaScript?

A high-level, interpreted, dynamically typed programming language.

Is JavaScript single-threaded?

Yes, it runs on a single call stack.

What is the event loop?

A mechanism that handles asynchronous operations by pushing callbacks from the task queue to the call stack.

JavaScript runtime

Environment that executes JS (Browser, Node.js).

Why JS is non-blocking

Because async tasks are handled via Web APIs and the event loop.