Syntax
new Intl.NumberFormat(locale, options).format(value)Examples
Formatting Numbers and Currency
Correctly formatted numbers respecting locale conventions.
const number = 1234567.891;
console.log(new Intl.NumberFormat("en-US").format(number));
// "1,234,567.891"
console.log(new Intl.NumberFormat("de-DE").format(number));
// "1.234.567,891" - different separators for German locale
const price = new Intl.NumberFormat("en-GB", {
style: "currency", currency: "GBP"
});
console.log(price.format(49.99)); // "£49.99"Formatting Dates
Locale-aware date formatting without manual string building.
const date = new Date(2026, 6, 20); // July 20, 2026
console.log(new Intl.DateTimeFormat("en-US").format(date));
// "7/20/2026"
console.log(new Intl.DateTimeFormat("en-GB").format(date));
// "20/07/2026"
const formatter = new Intl.DateTimeFormat("en-US", {
weekday: "long", year: "numeric", month: "long", day: "numeric"
});
console.log(formatter.format(date)); // "Monday, July 20, 2026"Best practices
- Use Intl.NumberFormat/DateTimeFormat instead of manually building formatted strings - locale conventions (separators, currency symbols, date order) vary in ways that are easy to get wrong by hand
- Reuse a single formatter instance for repeated formatting calls rather than creating a new one every time, for better performance
- Detect the user's locale with navigator.language rather than hardcoding one, for genuinely internationalized applications
- Use the currency and style options for money values rather than manually prepending a currency symbol
At a glance
- Purpose
- Application logic and interaction
- File extension
- .js · .mjs
- Runs in
- Browsers and JavaScript runtimes
- Usually used with
- HTML, CSS and Web APIs
Specifications & further reading
Related JavaScript documentation
Arrow Functions
Arrow functions provide a more concise syntax for writing function expressions in JavaScript. Introduced in ES6, they use the => syntax and have some important differences from regular functions, particularly in how they handle the "this" keyword. Arrow functions are especially useful for callbacks, array methods, and short inline functions.Variables: var, let, const
JavaScript has three ways to declare variables. var is the original way, function-scoped and hoisted with a default value of undefined. let, introduced in ES6, is block-scoped and can be reassigned. const is also block-scoped but cannot be reassigned after its initial value is set. Modern JavaScript strongly favors let and const over var.Data Types
JavaScript has seven primitive data types - string, number, boolean, undefined, null, bigint, and symbol - plus the object type, which includes arrays, functions, and plain objects. JavaScript is dynamically typed, meaning a variable can hold any type and that type can change. The typeof operator reports a value's type at runtime.Arithmetic Operators
Arithmetic operators perform mathematical calculations on numbers: addition (+), subtraction (-), multiplication (*), division (/), remainder/modulo (%), and exponentiation (**). The + operator also performs string concatenation when either operand is a string. Increment (++) and decrement (--) adjust a variable by one.
Arrow functions provide a more concise syntax for writing function expressions in JavaScript. Introduced in ES6, they use the => syntax and have some important differences from regular functions, particularly in how they handle the "this" keyword. Arrow functions are especially useful for callbacks, array methods, and short inline functions.Variables: var, let, const
JavaScript has three ways to declare variables. var is the original way, function-scoped and hoisted with a default value of undefined. let, introduced in ES6, is block-scoped and can be reassigned. const is also block-scoped but cannot be reassigned after its initial value is set. Modern JavaScript strongly favors let and const over var.Data Types
JavaScript has seven primitive data types - string, number, boolean, undefined, null, bigint, and symbol - plus the object type, which includes arrays, functions, and plain objects. JavaScript is dynamically typed, meaning a variable can hold any type and that type can change. The typeof operator reports a value's type at runtime.Arithmetic Operators
Arithmetic operators perform mathematical calculations on numbers: addition (+), subtraction (-), multiplication (*), division (/), remainder/modulo (%), and exponentiation (**). The + operator also performs string concatenation when either operand is a string. Increment (++) and decrement (--) adjust a variable by one.