JavaScript glossary

279 JavaScript terms, from Novice to Master. Start with the basics or jump to a level.

Updated October 19, 2026

Novice

<script> Element: An HTML element that loads or contains JavaScript for a web page.
Arithmetic Operators: The math symbols: + - * / // % **.
Arithmetic Operators (JavaScript): 3 + 4 // addition
Array - Access Element - JavaScript: Array indexes are 0 based.
Array - JavaScript: An array is a collection of multiple values in a single variable.
Assignment: Storing a value in a variable with a single =.
BigInt: A numeric primitive for integers larger than Number can safely represent.
Block Scope: The region inside braces where let and const declarations are available.
Browser Console: A browser development panel for inspecting messages, errors, and JavaScript values.
camelCase: A naming style that starts lowercase and capitalizes each later word.
Character - JavaScript: A character is a single letter, digit, punctuation or any other special character.
Comment: Text written for people reading the code that the JavaScript engine ignores.
Comparison Operators: Symbols like == and < that compare two values and answer True or False.
console.log(): A method that writes a value to the runtime's development console.
const: A keyword that declares a block-scoped variable that can't be given a new value.
ECMAScript: The language standard that defines how JavaScript syntax and behavior work.
else Statement: A branch that runs when its matching if condition is falsy.
Falsy: A value treated as false when JavaScript evaluates it as a condition.
if Statement: Runs a block of code only when a condition is true.
JavaScript: A programming language used to add behavior to web pages and build applications in browsers, servers, and other runtimes.
JavaScript Data Types: JavaScript basic data types are string, number and boolean.
JavaScript Engine: Software that reads and executes JavaScript code.
JavaScript Runtime: An environment combining a JavaScript engine with APIs such as timers, files, or browser features.
JavaScript variable: A variable is a named alias for a value that you want to use in your application.
let: A keyword that declares a block-scoped variable that can be reassigned.
Logical Operators: and, or and not: they combine True/False checks.
Naming Conventions (JavaScript): JavaScript predominantly uses 2 naming conventions: camelCase and PascalCase.
NaN: A special Number value meaning the result is not a valid number.
Node.js: A JavaScript runtime commonly used for servers, command-line tools, and automation.
null: A primitive value deliberately used to represent no object or value.
Number: JavaScript's main numeric type for both whole and fractional numbers.
Primitive Value: A value that is not an object, such as a string, number, boolean, null, or undefined.
Runtime Error: A problem that occurs while a compiled program is running.
Script: A file or block of JavaScript code that a browser or another runtime can execute.
Semicolon: A punctuation mark that can end a JavaScript statement.
Snake Case: All letters are lower case, each word is separated by an underscore.
Strict Equality (===): A comparison that requires equal values and matching types without coercion.
String: Text inside quotes, like \"hello\" or 'coffee'.
String - JavaScript: A string is a series of characters (letters and/or digits).
String Interpolation: Placing an expression inside ${} so its value becomes part of a template literal.
switch Statement: A statement that selects a code path by matching one value against cases.
Syntax Error: An error caused by code that does not follow JavaScript's grammar.
Template Literal: A string enclosed by backticks that supports interpolation and multiple lines.
Truthy: A value treated as true when JavaScript evaluates it as a condition.
Type Coercion: JavaScript's conversion of a value from one type to another, either automatically or explicitly.
typeof Operator: An operator that returns a string describing a value's basic type.
undefined: The primitive value used when a value has not been assigned or is missing.
var: An older keyword for declaring a variable that belongs to the whole function, not just the block it's in.
Variable: A name that holds a value, like a labelled box.
Variable Declaration: A statement that creates a variable name with let, const, or var.

Apprentice

Argument: The actual value you hand to a function when you call it.
Arguments (JavaScript): An argument is any value that is passed into a function when calling the function.
Array: A fixed-size, indexed collection whose elements all have the same type.
Array - Mixed Types - JavaScript: TRUE - JavaScript arrays can contain multiple types in the same array.
Array Destructuring: Syntax that extracts array items by position into variables.
Array Index: A zero-based number that identifies an element's position in an array.
Array Length: The fixed number of elements an array can hold.
Arrow Function: A compact function expression written with the => symbol.
Bracket Notation: Accessing an object property with a key expression inside square brackets.
break: Stops a loop immediately.
Callback Function: A function passed to other code to be called later or during an operation.
continue: Skips to the next pass of the loop.
Date Object: A built-in object representing a single moment in time.
Default Parameter: A parameter value used when the caller supplies undefined or omits the argument.
describe, it and expect: Group tests, define tests, and assert results.
do...while Loop: A loop that runs its body once before checking whether to repeat.
Dot Notation: Accessing an object property with a dot followed by its name.
Escape Sequence: A backslash code that represents a special character inside a string or character literal.
Event - JavaScript: An event is an any action that can happen that your application can monitor.
for Loop: Repeats code once for each item in a collection.
for...in Loop: A loop that visits an object's enumerable property names.
for...of Loop: A loop that visits the values produced by an iterable.
Function: A reusable block of code that can receive values and return a result.
Function Declaration: A named function created with the function keyword as a statement.
Function Expression: A function created as part of an expression and commonly assigned to a variable.
Function Signature (JavaScript): Functions have a signature, this is how the JavaScript runtime knows what code block to…
Global Scope: The outermost scope, whose declarations may be available across a script or module.
How to call a function: A function is called by specifying the name of the function followed by parentheses and…
includes(): A method that reports whether an array contains a value or a string contains text.
Jest: A popular JavaScript test framework.
join(): Glues a list of strings into one string with a separator between them.
JSON Object: A built-in object with methods for converting between JavaScript values and JSON text.
Key-Value Pair: One entry in a dictionary: a key and the value stored under it.
Local Scope: A function or block scope where declarations are available only inside that area.
Looping Logic
Math Object: A built-in object containing mathematical constants and functions.
Math.random(): A method that returns a pseudorandom double from 0.0 up to, but not including, 1.0.
Method: A function that belongs to a class, called on an object with a dot.
Nested Array: An array containing one or more arrays as its items.
Nested Loop: A loop inside another loop.
Nullish Coalescing: The ?? operator, which uses a fallback only when its left value is null or undefined.
Object: A class instance that combines its own state with available behavior.
Object - JavaScript: A JavaScript object gives you the ability to store multiple related variables in the same…
Object Destructuring: Syntax that extracts named object properties into variables.
Object Literal: Brace syntax that creates an object from a list of properties.
Optional Chaining: The ?. operator, which stops and returns undefined when the value before it is nullish.
Parameter: A name in a function definition for a value the function needs.
Parameter (JavaScript): A parameter (also known as an input parameter) is a way of receiving a variable into a…
Parameters and Arguments (JavaScript): A parameter is defined by the function to specify what input is required to call the…
parseFloat(): A function that reads a decimal number from the beginning of a string.
parseInt(): A function that reads a whole number from the beginning of a string.
pop(): Removes an item from a list and hands it back (the last one by default).
Property: A member that exposes a value through get and set accessors.
push(): An Array method that adds values to the end and returns the new length.
Rest Parameter: A final function parameter beginning with ... that collects remaining arguments into an array.
Return Value: The result a function hands back with return.
Return Value (JavaScript): Functions may or may not return a response value when they are called.
Scope: Where in the code a variable can be used.
shift(): An Array method that removes and returns the first item.
slice(): A method that returns a selected portion without changing the original array or string.
splice(): An Array method that removes, replaces, or inserts items by changing the original array.
split(): Breaks a string into a list of pieces.
Spread Syntax: The ... syntax that expands values from an iterable or properties from an object.
String Concatenation: Joining strings end to end with +.
String Index: A zero-based position used to locate a character in a String.
String Length: The number of UTF-16 code units in a String.
String Parsing: Strings cannot be directly converted to another data type, but strings can represent…
Ternary Operator: The ?: operator, which chooses one of two values from a condition.
The DOM: A browsers in-memory representation of the HTML document.
this: A value supplied by the way a regular function is called, often referring to the receiving object.
toString(): A method that returns a readable text representation of an object.
unshift(): An Array method that adds values to the beginning and returns the new length.
Vitest: A fast JavaScript test runner built on Vite.
while Loop: Keeps repeating as long as its condition stays true.

Journeyman

.catch(): A Promise method that schedules error handling for rejection.
.then(): A Promise method that schedules work for successful completion.
Array Iterator Method: An Array method that processes items through a callback, such as map, filter, or find.
async: A keyword added to functions that allows fetching data in parallel.
async Function: A function that always returns a Promise and allows await in its body.
await: An operator that pauses an async function until a value or Promise settles.
catch Block: A block that handles a matching exception from its try block.
Class: A blueprint for making objects that bundle data with actions.
Class Method: A function defined as a member of a JavaScript class.
Closure: An inner function that remembers the variables around it.
Constructor: The special class method that initializes a newly created JavaScript object.
Custom Error: An application-specific error class extending JavaScript's Error class.
Default Export: The single value a module marks as its default export.
defer: A Boolean classic-script attribute that delays execution until parsing finishes while preserving document order.
Dependency: External code that a project needs to compile or run.
Development Dependency: A package needed for development or testing but not for the application at runtime.
DOM: The browser's object representation of an HTML document.
Encapsulation: Keeping an object's data behind its methods so it's changed the safe way.
Error: An object describing a problem that interrupted normal execution.
ES Module: A JavaScript file using import and export to declare dependencies and public values.
Event: A member that lets an object notify subscribed code that something happened.
Event Listener: A callback registered to run when a particular event occurs.
every(): An Array method that reports whether every item passes a test.
extends: A keyword that makes one class inherit from another.
External script: A script loaded from a separate resource through a script element's src attribute.
Fetch API: A browser and runtime API for making HTTP requests that returns a Promise.
filter(): Keeps only the items that pass a test.
finally Block: A block that normally runs after try and catch whether an exception occurred or not.
find(): An Array method that returns the first item satisfying a test.
Getter: A method that returns an object's stored value.
Higher-Order Function: A function that takes or returns another function.
Inheritance: A new class starts with everything from another class, then adds or changes things.
Inline script: Script code written directly inside a script element.
Instance: One object made from a class, with its own data.
JSON.parse(): A method that converts valid JSON text into a JavaScript value.
JSON.stringify(): A method that converts a supported JavaScript value into JSON text.
Lexical Scope: The rule that a function can use names from where it was written.
map(): An Array method that creates a new array by transforming every item.
Method Overriding: A child class writing its own version of an inherited method.
Module script: A script processed as a JavaScript module with imports, strict mode, and module scoping.
Named Export: An exported value imported by its declared export name.
npm: The package registry and command-line client commonly used by JavaScript projects.
Package: A folder of related modules you can import from.
package.json: A project file containing package metadata, scripts, dependencies, and configuration.
Polymorphism: Different objects answering the same method call in their own way.
Private Field: A class field beginning with # that is accessible only inside its declaring class.
Promise: An object representing the eventual fulfillment or rejection of an asynchronous operation.
Promise State: A Promise's current state: pending, fulfilled, or rejected.
Pure Function: A function that produces the same result for the same inputs without changing outside state.
querySelector(): A DOM method that returns the first element matching a CSS selector.
Recursion: A function calling itself to solve a smaller piece of the same problem.
reduce(): A stream operation that combines elements into one result.
Response Object: An object returned by fetch containing an HTTP response's status, headers, and body methods.
Semantic Versioning: A major.minor.patch numbering convention communicating compatibility and change size.
Setter: A method that validates or changes an object's stored value.
some(): An Array method that reports whether at least one item passes a test.
sort(): Puts a list in order, changing the list itself.
Static Import: An import declaration resolved and loaded as part of a module's dependency graph.
Static Method: A class method called on the class itself rather than an instance.
super: A reference used to access a superclass constructor or member.
throw: A statement that deliberately raises an exception object.
try Block: A block containing code that may throw an exception.

Freelancer

<slot>: The element that marks where assigned light-DOM content appears inside a shadow tree.
<template>: The element that stores inert HTML fragments for later use.
AbortController: An object that sends cancellation signals to APIs such as fetch.
Async Iterator: An iterator whose next() method returns a Promise for each result.
Autonomous custom element: A custom element with its own hyphenated tag name.
Babel: A JavaScript compiler commonly used to transform newer syntax for older environments.
CommonJS: The Node.js module format based on require() and module.exports.
Currying: Transforming a function with several parameters into a sequence of one-parameter functions.
Custom element: An author-defined HTML element whose behavior is registered through the Custom Elements API.
Customized built-in element: A custom element that extends a built-in HTML element and uses the is attribute.
Cypress: A popular in-browser end-to-end testing tool.
Debounce: A technique that delays work until rapid calls have stopped for a chosen time.
Declarative shadow DOM: HTML syntax that creates a shadow root from a template element during parsing.
Deep Copy: A copy of a collection and everything inside it, fully separate.
Dynamic Import: The import() expression, which loads a module asynchronously when requested.
ESLint: A configurable tool that reports problems and style issues in JavaScript code.
Event Bubbling: The phase in which an event moves from its target up through ancestor elements.
Event Capturing: The phase in which an event travels from ancestors down toward its target.
Event Delegation: Handling events for many descendants with one listener on a shared ancestor.
Event Object: An object containing information about an event that occurred.
Event Target: The object on which an event was originally dispatched.
for await...of: A loop that awaits and visits values from an async iterable.
Generator Function: A function declared with function* that can pause and yield several values.
Immutability: The practice of creating new values instead of changing existing ones.
Iterable: Anything you can loop over with for.
Iterator: Gives out items one at a time with next() and remembers where it is.
Iterator Protocol: The next() method contract used to produce a sequence of values.
Local Storage: Browser key-value storage that remains after the page and browser are closed.
Map: A collection that associates unique keys with values.
Memoization: Caching a function's result so repeated inputs can return it without recalculation.
Mock Function: A test function that records calls and can return controlled values.
Module Bundler: A tool that combines a module graph into files prepared for deployment.
Named slot: A slot identified by name so matching host children can be assigned to it.
preventDefault(): An Event method that cancels the browser's default action when the event is cancelable.
Promise.all(): A method returning a Promise that fulfills when every input fulfills or rejects when one rejects.
Promise.allSettled(): A method returning results for every input after all have settled.
Promise.any(): A method returning the first fulfilled input or rejecting if all inputs reject.
Promise.race(): A method returning a Promise that settles when the first input settles.
Regular Expression: A pattern for finding or checking text.
Session Storage: Browser key-value storage kept for one tab's browsing session.
Set: A collection with no duplicates and no set order.
Shadow host: An element to which a shadow root is attached.
Shadow root: A document fragment that forms the root of a component's encapsulated shadow tree.
Shallow Copy: A new collection that still shares the items inside it with the original.
Slot assignment: The process that matches a shadow host's children with slots in its shadow tree.
Source Map: A file connecting transformed code locations back to the original source.
stopPropagation(): An Event method that stops the event from moving to other elements.
Strict Mode: A JavaScript mode that rejects some error-prone behavior and changes certain runtime rules.
structuredClone(): A function that makes a deep copy of many built-in JavaScript values.
Symbol: A primitive whose values are always unique and can be used as object property keys.
Throttle: A technique that limits how often repeated calls may run.
Transpiler: A tool that converts source code into another version of source code.
Tree Shaking: Removing exports that a build can determine are unused.
Unit Test: A small automated check of one focused unit of behavior.
WeakMap: A Map-like collection with weakly held object or non-registered Symbol keys.
WeakSet: A Set-like collection holding objects weakly so it does not prevent garbage collection.
Web Worker: A browser worker that runs JavaScript away from the page's main thread.
yield: Hands back one value from a generator and pauses until the next one is needed.

Master

Abstract Syntax Tree: A tree representation of source-code structure used by parsers and tools.
Accessor Property: An object property defined by getter and setter functions.
Call Stack: The stack of function calls currently being executed.
Comment node: A DOM node containing the text of an HTML comment.
Data Property: An object property that stores a value directly.
Document node: The DOM node representing the whole document and serving as the tree's root.
Element node: A DOM node representing an element and its attributes.
Enumerability: A property setting that controls whether common enumeration operations include that property.
Event Loop: The runtime mechanism that schedules queued work when the call stack is available.
Execution Context: The runtime state used to execute a script or function call.
FinalizationRegistry: An advanced API for requesting a callback after an object is garbage-collected.
Garbage Collection: Automatic recovery of heap memory from objects a program can no longer reach.
Hidden Class: An engine optimization that internally groups objects with the same property layout.
Hoisting: The effect of declarations being registered before execution reaches their source line.
Inherited Property: A property found through an object's prototype chain.
Inline Cache: An engine optimization that remembers prior property lookup or call results at a code location.
Just-in-Time Compilation: Runtime compilation of frequently executed JavaScript into optimized machine code.
Lexical Environment: An internal record connecting names in one scope to values and an outer environment.
Macrotask: An informal name for an event-loop task such as a timer or user event.
Memory Leak: Unwanted memory growth caused by retaining references to objects no longer needed.
Microtask Queue: A high-priority queue used by Promise reactions and queueMicrotask callbacks.
Node document order: The order nodes appear when a document tree is traversed from beginning to end.
Own Property: A property defined directly on an object rather than inherited.
Property Descriptor: A record controlling a property's value, accessors, writability, enumerability, and configurability.
Prototype: An object another object consults when a requested property is missing.
Prototype Chain: The linked sequence of prototypes searched during property lookup.
Proxy Object: An object that intercepts operations such as reading, writing, and property lookup for another object.
Realm: A JavaScript execution environment with its own global object and built-in objects.
Reflect API: Built-in methods that perform fundamental object operations and pair naturally with Proxy traps.
Run-to-Completion: The rule that one JavaScript job finishes before another job begins on the same agent.
Scope Chain: The sequence of lexical environments searched to resolve a name.
Task Queue: A queue of callbacks waiting to run as event-loop tasks.
Temporal Dead Zone: The period where a block-scoped variable exists but cannot be accessed before its declaration.
Text node: A DOM node that directly holds a string of characters.
Weak Reference: A reference to an object that does not prevent that object from being garbage-collected.

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