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Modern JavaScript Toolkit

This file is the practical ES6+ revision sheet: syntax, patterns, and APIs that modern JavaScript code uses every day.

TL;DR

Modern JavaScript improves readability, safer scoping, and better composition. Interviewers expect you to be comfortable with these features, not just recognize them.

let, const, And Scope

What To Know

  • let is block-scoped and reassignable.
  • const is block-scoped and cannot be rebound.
  • var is function-scoped and should be treated as legacy in modern code.

Arrow Functions

What To Know

  • Shorter syntax
  • Lexical this
  • Good for callbacks
  • Not suitable when you need dynamic this

Destructuring

Example

const user = { name: "Ada", age: 28 };
const { name, age } = user;

const point = [10, 20];
const [x, y] = point;

Rest And Spread

Example

const numbers = [1, 2, 3];
const copy = [...numbers];

function sum(...values) {
	return values.reduce((total, value) => total + value, 0);
}

Template Literals

Example

const name = "Ada";
const message = `Hello, ${name}`;

Default Parameters

function greet(name = "Guest") {
	return `Hello, ${name}`;
}

Modules

What To Know

  • export and import are standard module syntax.
  • Named exports and default exports solve different needs.
  • Modules improve dependency clarity.

Array Helpers

Know when and why to use:

  • map
  • filter
  • reduce
  • find
  • some
  • every
  • flatMap

Map And Set

Good Use Cases

  • Map for key-value storage where keys are not limited to strings.
  • Set for uniqueness.

Optional Chaining And Nullish Coalescing

Example

const city = user?.address?.city ?? "Unknown";

Common Trap

Do not confuse ?? with ||. ?? only falls back on null or undefined.

Symbols, Iterators, And Generators

What To Know

  • Symbol creates unique identifiers.
  • Iterators define how values are produced one by one.
  • Generators pause and resume execution with yield.

These appear less often in everyday frontend work, but still come up in deeper language interviews.

Rapid-Fire Questions

Why prefer const?

It prevents rebinding and makes intent clearer.

When is an arrow function a bad fit?

When you need a dynamic this, such as certain object methods or constructor-like behavior.

Why use Map over object?

When you need non-string keys, predictable iteration, or dedicated key-value semantics.