Let's start off by looking at the building blocks of any language: the types. JavaScript programs manipulate values, and those values all belong to a type. JavaScript offers sevenprimitive types:

• Number: used for all number values (integer and floating point) except forvery big integers.
• BigInt: used for arbitrarily large integers.
• String: used to store text.
• Boolean:true andfalse — usually used for conditional logic.
• Symbol: used for creating unique identifiers that won't collide.
• Undefined: indicating that a variable has not been assigned a value.
• Null: indicating a deliberate non-value.

Everything else is known as anObject. Common object types include:

• Function
• Array
• Map
• RegExp
• Error

Functions aren't special data structures in JavaScript — they are just a special type of object that can be called.

JavaScript has two built-in numeric types: Number and BigInt.

The Number type is aIEEE 754 64-bit double-precision floating point value, which means integers can be safely represented between-(2⁵³ − 1) and2⁵³ − 1 without loss of precision, and floating point numbers can be stored all the way up to1.79 × 10³⁰⁸. Within numbers, JavaScript does not distinguish between floating point numbers and integers.

console.log(3 / 2); // 1.5, not 1

So anapparent integer is in factimplicitly a float. Because of IEEE 754 encoding, sometimes floating point arithmetic can be imprecise.

console.log(0.1 + 0.2); // 0.30000000000000004

For operations that expect integers, such as bitwise operations, the number will be converted to a 32-bit integer.

Number literals can also have prefixes to indicate the base (binary, octal, decimal, or hexadecimal), or an exponent suffix.

console.log(0b111110111); // 503console.log(0o767); // 503console.log(0x1f7); // 503console.log(5.03e2); // 503

TheBigInt type is an arbitrary length integer. Its behavior is similar to C's integer types (e.g., division truncates to zero), except it can grow indefinitely. BigInts are specified with a number literal and ann suffix.

console.log(-3n / 2n); // -1n

The standardarithmetic operators are supported, including addition, subtraction, remainder arithmetic, etc. BigInts and numbers cannot be mixed in arithmetic operations.

TheMath object provides standard mathematical functions and constants.

Math.sin(3.5);const circumference = 2 * Math.PI * r;

There are three ways to convert a string to a number:

• parseInt(), which parses the string for an integer.
• parseFloat(), which parses the string for a floating-point number.
• TheNumber() function, which parses a string as if it's a number literal and supports many different number representations.

You can also use theunary plus+ as a shorthand forNumber().

Number values also includeNaN (short for "Not a Number") andInfinity. Many "invalid math" operations will returnNaN — for example, if attempting to parse a non-numeric string, or usingMath.log() on a negative value. Division by zero producesInfinity (positive or negative).

NaN is contagious: if you provide it as an operand to any mathematical operation, the result will also beNaN.NaN is the only value in JavaScript that's not equal to itself (per IEEE 754 specification).

Strings in JavaScript are sequences of Unicode characters. This should be welcome news to anyone who has had to deal with internationalization. More accurately, they areUTF-16 encoded.

console.log("Hello, world");console.log("你好，世界！"); // Nearly all Unicode characters can be written literally in string literals

Strings can be written with either single or double quotes — JavaScript does not have the distinction between characters and strings. If you want to represent a single character, you just use a string consisting of that single character.

console.log("Hello"[1] === "e"); // true

To find the length of a string (incode units), access itslength property.

Strings haveutility methods to manipulate the string and access information about the string. Because all primitives are immutable by design, these methods return new strings.

The+ operator is overloaded for strings: when one of the operands is a string, it performs string concatenation instead of number addition. A specialtemplate literal syntax allows you to write strings with embedded expressions more succinctly. Unlike Python's f-strings or C#'s interpolated strings, template literals use backticks (not single or double quotes).

const age = 25;console.log("I am " + age + " years old."); // String concatenationconsole.log(`I am ${age} years old.`); // Template literal

JavaScript distinguishes betweennull, which indicates a deliberate non-value (and is only accessible through thenull keyword), andundefined, which indicates absence of value. There are many ways to obtainundefined:

• Areturn statement with no value (return;) implicitly returnsundefined.
• Accessing a nonexistentobject property (obj.iDontExist) returnsundefined.
• A variable declaration without initialization (let x;) will implicitly initialize the variable toundefined.

JavaScript has a Boolean type, with possible valuestrue andfalse — both of which are keywords. Any value can be converted to a boolean according to the following rules:

1. false,0, empty strings (""),NaN,null, andundefined all becomefalse.
2. All other values becometrue.

You can perform this conversion explicitly using theBoolean() function:

Boolean(""); // falseBoolean(234); // true

However, this is rarely necessary, as JavaScript will silently perform this conversion when it expects a boolean, such as in anif statement (seeControl structures). For this reason, we sometimes speak of "truthy" and "falsy", meaning values that becometrue andfalse, respectively, when used in boolean contexts.

Boolean operations such as&& (logicaland),|| (logicalor), and! (logicalnot) are supported; seeOperators.

The Symbol type is often used to create unique identifiers. Every symbol created with theSymbol() function is guaranteed to be unique. In addition, there are registered symbols, which are shared constants, and well-known symbols, which are utilized by the language as "protocols" for certain operations. You can read more about them in thesymbol reference.

Variables in JavaScript are declared using one of three keywords:let,const, orvar.

let allows you to declare block-level variables. The declared variable is available from theblock it is enclosed in.

let a;let name = "Simon";// myLetVariable is *not* visible out herefor (let myLetVariable = 0; myLetVariable < 5; myLetVariable++) {  // myLetVariable is only visible in here}// myLetVariable is *not* visible out here

const allows you to declare variables whose values are never intended to change. The variable is available from theblock it is declared in.

const Pi = 3.14; // Declare variable Piconsole.log(Pi); // 3.14

A variable declared withconst cannot be reassigned.

const Pi = 3.14;Pi = 1; // will throw an error because you cannot change a constant variable.

const declarations only preventreassignments — they don't preventmutations of the variable's value, if it's an object.

const obj = {};obj.a = 1; // no errorconsole.log(obj); // { a: 1 }

var declarations can have surprising behaviors (for example, they are not block-scoped), and they are discouraged in modern JavaScript code.

If you declare a variable without assigning any value to it, its value isundefined. You can't declare aconst variable without an initializer, because you can't change it later anyway.

let andconst declared variables still occupy the entire scope they are defined in, and are in a region known as thetemporal dead zone before the actual line of declaration. This has some interesting interactions with variable shadowing, which don't occur in other languages.

function foo(x, condition) {  if (condition) {    console.log(x);    const x = 2;    console.log(x);  }}foo(1, true);

In most other languages, this would log "1" and "2", because before theconst x = 2 line,x should still refer to the parameterx in the upper scope. In JavaScript, because each declaration occupies the entire scope, this would throw an error on the firstconsole.log: "Cannot access 'x' before initialization". For more information, see the reference page oflet.

JavaScript isdynamically typed. Types (as described inthe previous section) are only associated with values, but not with variables. Forlet-declared variables, you can always change its type through reassignment.

let a = 1;a = "foo";

JavaScript's numeric operators include+,-,*,/,% (remainder), and** (exponentiation). Values are assigned using=. Each binary operator also has a compound assignment counterpart such as+= and-=, which extend out tox = x operator y.

x += 5;x = x + 5;

You can use++ and-- to increment and decrement respectively. These can be used as a prefix or postfix operators.

The+ operator also does string concatenation:

"hello" + " world"; // "hello world"

If you add a string to a number (or other value) everything is converted into a string first. This might trip you up:

"3" + 4 + 5; // "345"3 + 4 + "5"; // "75"

Adding an empty string to something is a useful way of converting it to a string itself.

Comparisons in JavaScript can be made using<,>,<= and>=, which work for both strings and numbers. For equality, thedouble-equals operator performs type coercion if you give it different types, with sometimes interesting results. On the other hand, thetriple-equals operator does not attempt type coercion, and is usually preferred.

123 == "123"; // true1 == true; // true123 === "123"; // false1 === true; // false

The double-equals and triple-equals also have their inequality counterparts:!= and!==.

JavaScript also hasbitwise operators andlogical operators. Notably, logical operators don't work with boolean values only — they work by the "truthiness" of the value.

const a = 0 && "Hello"; // 0 because 0 is "falsy"const b = "Hello" || "world"; // "Hello" because both "Hello" and "world" are "truthy"

The&& and|| operators use short-circuit logic, which means whether they will execute their second operand is dependent on the first. This is useful for checking for null objects before accessing their attributes:

const name = o && o.getName();

Or for caching values (when falsy values are invalid):

const name = cachedName || (cachedName = getName());

For a comprehensive list of operators, see theguide page orreference section. You may be especially interested in theoperator precedence.

JavaScript grammar is very similar to the C family. There are a few points worth mentioning:

• Identifiers can have Unicode characters, but they cannot be one of thereserved words.
• Comments are commonly// or/* */, while many other scripting languages like Perl, Python, and Bash use#.
• Semicolons are optional in JavaScript — the languageautomatically inserts them when needed. However, there are certain caveats to watch out for, since unlike Python, semicolons are still part of the syntax.

For an in-depth look at the JavaScript grammar, see thereference page for lexical grammar.

JavaScript has a similar set of control structures to other languages in the C family. Conditional statements are supported byif andelse; you can chain them together:

let name = "kittens";if (name === "puppies") {  name += " woof";} else if (name === "kittens") {  name += " meow";} else {  name += "!";}name === "kittens meow";

JavaScript doesn't haveelif, andelse if is really just anelse branch comprised of a singleif statement.

JavaScript haswhile loops anddo...while loops. The first is good for basic looping; the second is for loops where you wish to ensure that the body of the loop is executed at least once:

while (true) {  // an infinite loop!}let input;do {  input = get_input();} while (inputIsNotValid(input));

JavaScript'sfor loop is the same as that in C and Java: it lets you provide the control information for your loop on a single line.

for (let i = 0; i < 5; i++) {  // Will execute 5 times}

JavaScript also contains two other prominent for loops:for...of, which iterates overiterables, most notably arrays, andfor...in, which visits allenumerable properties of an object.

for (const value of array) {  // do something with value}for (const property in object) {  // do something with object property}

Theswitch statement can be used for multiple branches based on equality checking.

switch (action) {  case "draw":    drawIt();    break;  case "eat":    eatIt();    break;  default:    doNothing();}

Similar to C, case clauses are conceptually the same aslabels, so if you don't add abreak statement, execution will "fall through" to the next level. However, they are not actually jump tables — any expression can be part of thecase clause, not just string or number literals, and they would be evaluated one-by-one until one equals the value being matched. Comparison takes place between the two using the=== operator.

Unlike some languages like Rust, control-flow structures are statements in JavaScript, meaning you can't assign them to a variable, likeconst a = if (x) { 1 } else { 2 }.

JavaScript errors are handled using thetry...catch statement.

try {  buildMySite("./website");} catch (e) {  console.error("Building site failed:", e);}

Errors can be thrown using thethrow statement. Many built-in operations may throw as well.

function buildMySite(siteDirectory) {  if (!pathExists(siteDirectory)) {    throw new Error("Site directory does not exist");  }}

In general, you can't tell the type of the error you just caught, because anything can be thrown from athrow statement. However, you can usually assume it's anError instance, as is the example above. There are some subclasses ofError built-in, likeTypeError andRangeError, that you can use to provide extra semantics about the error. There's no conditional catch in JavaScript — if you only want to handle one type of error, you need to catch everything, identify the type of error usinginstanceof, and then rethrow the other cases.

try {  buildMySite("./website");} catch (e) {  if (e instanceof RangeError) {    console.error("Seems like a parameter is out of range:", e);    console.log("Retrying...");    buildMySite("./website");  } else {    // Don't know how to handle other error types; throw them so    // something else up in the call stack may catch and handle it    throw e;  }}

If an error is uncaught by anytry...catch in the call stack, the program will exit.

For a comprehensive list of control flow statements, see thereference section.

JavaScript objects can be thought of as collections of key-value pairs. As such, they are similar to:

• Dictionaries in Python.
• Hashes in Perl and Ruby.
• Hash tables in C and C++.
• HashMaps in Java.
• Associative arrays in PHP.

JavaScript objects are hashes. Unlike objects in statically typed languages, objects in JavaScript do not have fixed shapes — properties can be added, deleted, re-ordered, mutated, or dynamically queried at any time. Object keys are alwaysstrings orsymbols — even array indices, which are canonically integers, are actually strings under the hood.

Objects are usually created using the literal syntax:

const obj = {  name: "Carrot",  for: "Max",  details: {    color: "orange",    size: 12,  },};

Object properties can beaccessed using dot (.) or square brackets ([]). When using the dot notation, the key must be a valididentifier. Square brackets, on the other hand, allow indexing the object with a dynamic key value.

// Dot notationobj.name = "Simon";const name = obj.name;// Bracket notationobj["name"] = "Simon";const name = obj["name"];// Can use a variable to define a keyconst userName = prompt("what is your key?");obj[userName] = prompt("what is its value?");

Property access can be chained together:

obj.details.color; // orangeobj["details"]["size"]; // 12

Objects are always references, so unless something is explicitly copying the object, mutations to an object would be visible to the outside.

const obj = {};function doSomething(o) {  o.x = 1;}doSomething(obj);console.log(obj.x); // 1

This also means two separately created objects will never be equal (!==), because they are different references. If you hold two references of the same object, mutating one would be observable through the other.

const me = {};const stillMe = me;me.x = 1;console.log(stillMe.x); // 1

For more on objects and prototypes, see theObject reference page. For more information on the object initializer syntax, see itsreference page.

This page has omitted all details about object prototypes and inheritance because you can usually achieve inheritance withclasses without touching the underlying mechanism (which you may have heard to be abstruse). To learn about them, seeInheritance and the prototype chain.

Arrays in JavaScript are actually a special type of object. They work very much like regular objects (numerical properties can naturally be accessed only using[] syntax) but they have one magic property calledlength. This is always one more than the highest index in the array.

Arrays are usually created with array literals:

const a = ["dog", "cat", "hen"];a.length; // 3

JavaScript arrays are still objects — you can assign any properties to them, including arbitrary number indices. The only "magic" is thatlength will be automatically updated when you set a particular index.

const a = ["dog", "cat", "hen"];a[100] = "fox";console.log(a.length); // 101console.log(a); // ['dog', 'cat', 'hen', empty × 97, 'fox']

The array we got above is called asparse array because there are uninhabited slots in the middle, and will cause the engine to deoptimize it from an array to a hash table. Make sure your array is densely populated!

Out-of-bounds indexing doesn't throw. If you query a non-existent array index, you'll get a value ofundefined in return:

const a = ["dog", "cat", "hen"];console.log(typeof a[90]); // undefined

Arrays can have any elements and can grow or shrink arbitrarily.

const arr = [1, "foo", true];arr.push({});// arr = [1, "foo", true, {}]

Arrays can be iterated with thefor loop, as you can in other C-like languages:

for (let i = 0; i < a.length; i++) {  // Do something with a[i]}

Or, since arrays are iterable, you can use thefor...of loop, which is synonymous to C++/Java'sfor (int x : arr) syntax:

for (const currentValue of a) {  // Do something with currentValue}

Arrays come with a plethora ofarray methods. Many of them would iterate the array — for example,map() would apply a callback to every array element, and return a new array:

const babies = ["dog", "cat", "hen"].map((name) => `baby ${name}`);// babies = ['baby dog', 'baby cat', 'baby hen']

Along with objects, functions are the core component in understanding JavaScript. The most basic function declaration looks like this:

function add(x, y) {  const total = x + y;  return total;}

A JavaScript function can take 0 or more parameters. The function body can contain as many statements as you like and can declare its own variables which are local to that function. Thereturn statement can be used to return a value at any time, terminating the function. If no return statement is used (or an empty return with no value), JavaScript returnsundefined.

Functions can be called with more or fewer parameters than it specifies. If you call a function without passing the parameters it expects, they will be set toundefined. If you pass more parameters than it expects, the function will ignore the extra parameters.

add(); // NaN// Equivalent to add(undefined, undefined)add(2, 3, 4); // 5// added the first two; 4 was ignored

There are a number of other parameter syntaxes available. For example, therest parameter syntax allows collecting all the extra parameters passed by the caller into an array, similar to Python's*args. (Since JS doesn't have named parameters on the language level, there's no**kwargs.)

function avg(...args) {  let sum = 0;  for (const item of args) {    sum += item;  }  return sum / args.length;}avg(2, 3, 4, 5); // 3.5

In the above code, the variableargs holds all the values that were passed into the function.

The rest parameter will store all argumentsafter where it's declared, but not before. In other words,function avg(firstValue, ...args) will store the first value passed into the function in thefirstValue variable and the remaining arguments inargs.

If a function accepts a list of arguments and you already hold them in an array, you can use thespread syntax in the function call tospread the array as a list of elements. For instance:avg(...numbers).

We mentioned that JavaScript doesn't have named parameters. It's possible, though, to implement them usingobject destructuring, which allows objects to be conveniently packed and unpacked.

// Note the { } braces: this is destructuring an objectfunction area({ width, height }) {  return width * height;}// The { } braces here create a new objectconsole.log(area({ width: 2, height: 3 }));

There's also thedefault parameter syntax, which allows omitted parameters (or those passed asundefined) to have a default value.

function avg(firstValue, secondValue, thirdValue = 0) {  return (firstValue + secondValue + thirdValue) / 3;}avg(1, 2); // 1, instead of NaN

JavaScript lets you create anonymous functions — that is, functions without names. In practice, anonymous functions are typically used as arguments to other functions, immediately assigned to a variable that can be used to invoke the function, or returned from another function.

// Note that there's no function name before the parenthesesconst avg = function (...args) {  let sum = 0;  for (const item of args) {    sum += item;  }  return sum / args.length;};

That makes the anonymous function invocable by callingavg() with some arguments — that is, it's semantically equivalent to declaring the function using thefunction avg() {} declaration syntax.

There's another way to define anonymous functions — using anarrow function expression.

// Note that there's no function name before the parenthesesconst avg = (...args) => {  let sum = 0;  for (const item of args) {    sum += item;  }  return sum / args.length;};// You can omit the `return` when simply returning an expressionconst sum = (a, b, c) => a + b + c;

Arrow functions are not semantically equivalent to function expressions — for more information, see itsreference page.

There's another way that anonymous functions can be useful: it can be simultaneously declared and invoked in a single expression, called anImmediately invoked function expression (IIFE):

(function () {  // …})();

For use-cases of IIFEs, you can reademulating private methods with closures.

JavaScript allows you to call functions recursively. This is particularly useful for dealing with tree structures, such as those found in the browser DOM.

function countChars(elm) {  if (elm.nodeType === 3) {    // TEXT_NODE    return elm.nodeValue.length;  }  let count = 0;  for (let i = 0, child; (child = elm.childNodes[i]); i++) {    count += countChars(child);  }  return count;}

Function expressions can be named as well, which allows them to be recursive.

const charsInBody = (function counter(elm) {  if (elm.nodeType === 3) {    // TEXT_NODE    return elm.nodeValue.length;  }  let count = 0;  for (let i = 0, child; (child = elm.childNodes[i]); i++) {    count += counter(child);  }  return count;})(document.body);

The name provided to a function expression as above is only available to the function's own scope. This allows more optimizations to be done by the engine and results in more readable code. The name also shows up in the debugger and some stack traces, which can save you time when debugging.

If you are used to functional programming, beware of the performance implications of recursion in JavaScript. Although the language specification specifiestail-call optimization, only JavaScriptCore (used by Safari) has implemented it, due to the difficulty of recovering stack traces and debuggability. For deep recursion, consider using iteration instead to avoid stack overflow.

JavaScript functions are first-class objects. This means that they can be assigned to variables, passed as arguments to other functions, and returned from other functions. In addition, JavaScript supportsclosures out-of-the-box without explicit capturing, allowing you to conveniently apply functional programming styles.

// Function returning functionconst add = (x) => (y) => x + y;// Function accepting functionconst babies = ["dog", "cat", "hen"].map((name) => `baby ${name}`);

Note that JavaScript functions are themselves objects — like everything else in JavaScript — and you can add or change properties on them just like we've seen earlier in the Objects section.

JavaScript function declarations are allowed inside other functions. An important detail of nested functions in JavaScript is that they can access variables in their parent function's scope:

function parentFunc() {  const a = 1;  function nestedFunc() {    const b = 4; // parentFunc can't use this    return a + b;  }  return nestedFunc(); // 5}

This provides a great deal of utility in writing more maintainable code. If a called function relies on one or two other functions that are not useful to any other part of your code, you can nest those utility functions inside it. This keeps the number of functions that are in the global scope down.

This is also a great counter to the lure of global variables. When writing complex code, it is often tempting to use global variables to share values between multiple functions, which leads to code that is hard to maintain. Nested functions can share variables in their parent, so you can use that mechanism to couple functions together without polluting your global namespace.

JavaScript offers theclass syntax that's very similar to languages like Java.

class Person {  constructor(name) {    this.name = name;  }  sayHello() {    return `Hello, I'm ${this.name}!`;  }}const p = new Person("Maria");console.log(p.sayHello());

JavaScript classes are just functions that must be instantiated with thenew operator. Every time a class is instantiated, it returns an object containing the methods and properties that the class specified. Classes don't enforce any code organization — for example, you can have functions returning classes, or you can have multiple classes per file. Here's an example of how ad hoc the creation of a class can be: it's just an expression returned from an arrow function. This pattern is called amixin.

const withAuthentication = (cls) =>  class extends cls {    authenticate() {      // …    }  };class Admin extends withAuthentication(Person) {  // …}

Static properties are created by prependingstatic. Private fields and methods are created by prepending a hash# (notprivate). The hash is an integral part of the element's name, and distinguishes it from a regular string-keyed property. (Think about# as_ in Python.) Unlike most other languages, there's absolutely no way to read a private element outside the class body — not even in derived classes.

For a detailed guide on various class features, you can read theguide page.

JavaScript is single-threaded by nature. There's noparalleling; onlyconcurrency. Asynchronous programming is powered by anevent loop, which allows a set of tasks to be queued and polled for completion.

There are three idiomatic ways to write asynchronous code in #"/en-US/docs/Web/API/Window/setTimeout" title="setTimeout()">setTimeout())

For example, here's what a file-read operation might look like in #"/en-US/docs/Web/API/Permissions_API">asking user permissions, tofetching data, toreading files. Keeping the potentially long-running operations async ensures that other processes can still run while this one waits — for example, the browser will not freeze while waiting for the user to click a button to grant permission.

If you have an async value, it's not possible to get its value synchronously. For example, if you have a promise, you can only access the eventual result via thethen() method. Similarly,await can only be used in an async context, which is usually an async function or a module. Promises arenever blocking — only the logic depending on the promise's result will be deferred; everything else continues to execute in the meantime. If you are a functional programmer, you may recognize promises asmonads which can be mapped withthen() (however, they are notproper monads because they auto-flatten; i.e., you can't have aPromise<Promise<T>>).

In fact, the single-threaded model has made Node.js a popular choice for server-side programming due to its non-blocking IO, making handling a large number of database or file-system requests very performant. However, CPU-bound (computationally intensive) tasks that are pure JavaScript will still block the main thread. To achieve real paralleling, you may need to useworkers.

To learn more about asynchronous programming, you can read aboutusing promises or follow theasynchronous JavaScript tutorial.

JavaScript also specifies a module system supported by most runtimes. A module is usually a file, identified by its file path or URL. You can use theimport andexport statements to exchange data between modules:

import { foo } from "./foo.js";// Unexported variables are local to the moduleconst b = 2;export const a = 1;

Unlike Haskell, Python, Java, etc., JavaScript module resolution is entirely host-defined — it's usually based on URLs or file paths, so relative file paths "just work" and are relative to the current module's path instead of some project root path.

However, the JavaScript language doesn't offer standard library modules — all core functionalities are powered by global variables likeMath andIntl instead. This is due to the long history of JavaScript lacking a module system, and the fact that opting into the module system involves some changes to the runtime setup.

Different runtimes may use different module systems. For example,Node.js uses the package managernpm and is mostly file-system based, whileDeno and browsers are fully URL-based and modules can be resolved from HTTP URLs.

For more information, see themodules guide page.

Throughout this page, we've constantly mentioned that certain features arelanguage-level while others areruntime-level.

JavaScript is a general-purpose scripting language. Thecore language specification focuses on pure computational logic. It doesn't deal with any input/output — in fact, without extra runtime-level APIs (most notablyconsole.log()), a JavaScript program's behavior is entirely unobservable.

A runtime, or a host, is something that feeds data to the JavaScript engine (the interpreter), provides extra global properties, and provides hooks for the engine to interact with the outside world. Module resolution, reading data, printing messages, sending network requests, etc. are all runtime-level operations. Since its inception, JavaScript has been adopted in various environments, such as browsers (which provide APIs likeDOM), Node.js (which provides APIs likefile system access), etc. JavaScript has been successfully integrated in web (which was its primary purpose), mobile apps, desktop apps, server-side apps, serverless, embedded systems, and more. While you learn about JavaScript core features, it's also important to understand host-provided features in order to put the knowledge to use. For example, you can read about allweb platform APIs, which are implemented by browsers, and sometimes non-browsers.

This page offers a very basic insight into how various JavaScript features compare with other languages. If you want to learn more about the language itself and the nuances of each feature, you can read theJavaScript guide and theJavaScript reference.

There are some essential parts of the language that we have omitted due to space and complexity, but you can explore on your own:

• Inheritance and the prototype chain
• Closures
• Regular expressions
• Iteration