Syntax
const city = user?.address?.city ?? "Unknown";Examples
Safely reading nested data
A small, runnable example of this syntax.
const user = { name: "Ada" };
const city = user.address?.city ?? "Unknown";
console.log(city); // UnknownBest practices
- Use ?? when 0, false, or an empty string are valid values; use || only when all falsey values should trigger the fallback.
- Keep examples small while learning, then combine the idea with a real project.
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.