Glossary

Javascript · Reference cheat sheet

📋 Overview

Alphabetical glossary of core JavaScript terms spanning values, functions, async flow, objects, and browser/runtime APIs.

🔧 Core concepts

Term	Definition
Array	An ordered, indexable collection of values with array methods.
Async/await	Syntax that pauses an async function until a Promise settles.
BigInt	An integer type for values beyond `Number.MAX_SAFE_INTEGER`.
Boolean	A primitive that is either `true` or `false`.
Callback	A function passed to another function to run later.
Closure	A function that retains access to variables from its outer scope.
Coercion	Automatic or explicit conversion of a value from one type to another.
Destructuring	Syntax that unpacks values from arrays or properties from objects.
Event loop	The runtime mechanism that schedules macrotasks and microtasks.
Fetch	The standard API for making HTTP requests that returns a Promise.
Function	A callable object that executes a block of code with parameters.
Generator	A function using `function*` and `yield` that produces an iterator.
Hoisting	Moving declarations to the top of their scope during compilation.
IIFE	Immediately Invoked Function Expression that runs as soon as it is defined.
Iterable	An object that implements `[Symbol.iterator]` and can be looped with `for...of`.
JSON	A text data format and global object for parse/stringify.
Map	A collection of key–value pairs that preserves insertion order.
Module	A file with its own scope that exports and imports bindings.
Null	A primitive representing intentional absence of an object value.
Object	A collection of named properties; almost everything non-primitive is one.
Optional chaining	The `?.` operator that short-circuits when a reference is nullish.
Promise	An object representing a future success or failure of an async operation.
Prototype	The object linked for inheritance lookups via the prototype chain.
Proxy	An object that intercepts fundamental operations on another object.
Scope	The region of code where a binding is visible.
Set	A collection of unique values.
Spread	The `...` syntax that expands iterables or object properties.
Strict mode	A restricted JavaScript variant that catches silent errors.
Symbol	A unique, immutable primitive often used as non-colliding property keys.
Template literal	A string delimited by backticks that supports interpolation and multiline text.
This	A binding whose value depends on how a function is called.
Undefined	A primitive meaning a variable has been declared but not assigned.
WeakMap	A Map-like collection whose keys are weakly held objects.

💡 Examples

Closure and destructuring:

function makeCounter() {
  let n = 0;
  return () => ++n;
}
const { length } = [1, 2, 3];

Promise with async/await:

async function loadUser(id) {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new Error("failed");
  return res.json();
}

Optional chaining and nullish coalescing:

const name = user?.profile?.name ?? "Anonymous";

⚠️ Pitfalls

Confusing null (intentional empty) with undefined (missing value).
Mixing == coercion equality with === strict equality.
Assuming this always means the enclosing object — it depends on call style.
Treating Map like a plain Object — keys and iteration differ.
Equating callback hell with Promise chains — both are async, different control styles.

Glossary

Glossary

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

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