Beginner JavaScript Interview Questions and Answers

JavaScript is a dynamically typed, high-level programming language and the scripting language of the web. It runs in every browser, where it makes pages interactive through the DOM, and outside the browser with runtimes such as Node.js (servers, tools), as well as in mobile and desktop frameworks. It is standardised as ECMAScript.

• var is function-scoped, can be redeclared, and is hoisted as undefined.
• let is block-scoped, can be reassigned, and throws a ReferenceError if used before its declaration (the temporal dead zone).
• const is block-scoped like let, but cannot be reassigned. Objects stored in a const can still be modified.

Use const by default, let when the value changes, and avoid var.

Seven primitives: string, number, bigint, boolean, undefined, null and symbol. Everything else is an object, including arrays, functions and dates. Primitives are immutable and copied by value; objects are passed around by reference.

=== (strict equality) compares type and value with no conversion. == (loose equality) converts types first, which gives surprising results:

console.log(0 == "", 0 === "");
console.log("5" == 5, "5" === 5);
console.log(null == undefined, null === undefined);

true false
true false
true false

Always use ===.

undefined means a value was never assigned: an unset variable, a missing argument or a non-existent property. null is assigned deliberately to mean "no value". typeof undefined is "undefined", while typeof null is "object" (a historic bug).

false, 0, -0, 0n, "", null, undefined and NaN. Everything else is truthy, including "0", "false", [] and {}.

typeof [] is "object", because arrays are objects. Use Array.isArray(value).

Strings written with backticks that support interpolation with ${expression} and span multiple lines: `Hello, ${name}! You have ${count} messages.`

A declaration (function add(a, b) {}) is hoisted, so it can be called before it appears in the code. An expression (const add = function (a, b) {} or an arrow function) is created when that line runs, so it cannot be called earlier.

Arrow functions have shorter syntax and an implicit return for single expressions. They do not have their own this (they use the this of the surrounding code), have no arguments object, and cannot be used with new. Use them for callbacks; use regular methods in objects and classes when you need this.

map returns a new array made from the callback's return values. forEach returns undefined and is used only for side effects such as logging.

const nums = [1, 2, 3];
console.log(nums.map((n) => n * 2));
console.log(nums.forEach((n) => n * 2));

[ 2, 4, 6 ]
undefined

Convert it to a Set and back:

console.log([...new Set([3, 1, 3, 2, 1])]);

[ 3, 1, 2 ]

An array is an ordered list accessed by numeric index (list[0]), with methods like push, map and filter. An object stores named properties accessed by key (user.name). Use arrays for collections of similar items and objects for describing a single thing.

NaN ("not a number") is a number value produced by invalid maths, such as "abc" * 2. It is not equal to anything, even itself, so NaN === NaN is false. Check with Number.isNaN(value).

It turns on strict mode, which turns silent mistakes into errors (assigning to undeclared variables, writing to read-only properties) and makes this undefined in plain function calls. Modules and classes are always strict.

Number("42") converts a whole string (giving NaN if invalid); parseInt("42px", 10) and parseFloat("3.5kg") read a leading number; unary +"42" is a shortcut for Number. Remember that form inputs always give strings.