File size: 7,546 Bytes
780c9fe |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 |
---
title: function
slug: Web/JavaScript/Reference/Statements/function
page-type: javascript-statement
browser-compat: javascript.statements.function
sidebar: jssidebar
---
The **`function`** declaration creates a {{Glossary("binding")}} of a new function to a given name.
You can also define functions using the [`function` expression](/en-US/docs/Web/JavaScript/Reference/Operators/function).
{{InteractiveExample("JavaScript Demo: function declaration", "shorter")}}
```js interactive-example
function calcRectArea(width, height) {
return width * height;
}
console.log(calcRectArea(5, 6));
// Expected output: 30
```
## Syntax
```js-nolint
function name(param0) {
statements
}
function name(param0, param1) {
statements
}
function name(param0, param1, /* …, */ paramN) {
statements
}
```
### Parameters
- `name`
- : The function name.
- `param` {{optional_inline}}
- : The name of a formal parameter for the function. Maximum number of arguments varies in different engines. For the parameters' syntax, see the [Functions reference](/en-US/docs/Web/JavaScript/Guide/Functions#function_parameters).
- `statements` {{optional_inline}}
- : The statements which comprise the body of the function.
## Description
A `function` declaration creates a {{jsxref("Function")}} object. Each time when a function is called, it returns the value specified by the last executed {{jsxref("Statements/return", "return")}} statement, or `undefined` if the end of the function body is reached. See [functions](/en-US/docs/Web/JavaScript/Reference/Functions) for detailed information on functions.
`function` declarations behave like a mix of {{jsxref("Statements/var", "var")}} and {{jsxref("Statements/let", "let")}}:
- Like `let`, in strict mode, [function declarations are scoped to the most closely containing block](#block-level_function_declaration).
- Like `let`, function declarations at the top level of a module or within blocks in strict mode cannot be [redeclared](#redeclarations) by any other declaration.
- Like `var`, function declarations at the top level of a script (strict or non-strict) become properties on {{jsxref("globalThis")}}. Function declarations at the top level of a script or function body (strict or non-strict) can be redeclared by another `function` or `var`.
- Like both, function declarations can be re-assigned, but you should avoid doing so.
- Unlike either, function declarations are [hoisted](#hoisting) together with its value and can be called anywhere in its scope.
### Block-level function declaration
> [!WARNING]
> In [non-strict mode](/en-US/docs/Web/JavaScript/Reference/Strict_mode), function declarations inside blocks behave strangely. Only declare functions in blocks if you are in strict mode.
Functions can be conditionally declared — that is, a function statement can be nested within an [`if`](/en-US/docs/Web/JavaScript/Reference/Statements/if...else) statement. However, in non-strict mode, the results are inconsistent across implementations.
```js
console.log(
`'foo' name ${
"foo" in globalThis ? "is" : "is not"
} global. typeof foo is ${typeof foo}`,
);
if (false) {
function foo() {
return 1;
}
}
// In Chrome:
// 'foo' name is global. typeof foo is undefined
//
// In Firefox:
// 'foo' name is global. typeof foo is undefined
//
// In Safari:
// 'foo' name is global. typeof foo is function
```
The scoping and hoisting effect won't change regardless of whether the `if` body is actually executed.
```js
console.log(
`'foo' name ${
"foo" in globalThis ? "is" : "is not"
} global. typeof foo is ${typeof foo}`,
);
if (true) {
function foo() {
return 1;
}
}
// In Chrome:
// 'foo' name is global. typeof foo is undefined
//
// In Firefox:
// 'foo' name is global. typeof foo is undefined
//
// In Safari:
// 'foo' name is global. typeof foo is function
```
In [strict mode](/en-US/docs/Web/JavaScript/Reference/Strict_mode), [block](/en-US/docs/Web/JavaScript/Reference/Statements/block)-level function declarations are scoped to that block and are hoisted to the top of the block.
```js
"use strict";
{
foo(); // Logs "foo"
function foo() {
console.log("foo");
}
}
console.log(
`'foo' name ${
"foo" in globalThis ? "is" : "is not"
} global. typeof foo is ${typeof foo}`,
);
// 'foo' name is not global. typeof foo is undefined
```
### Hoisting
Function declarations in JavaScript are [hoisted](/en-US/docs/Glossary/Hoisting) to the top of the enclosing function or global scope. You can use the function before you declared it:
```js
hoisted(); // Logs "foo"
function hoisted() {
console.log("foo");
}
```
Note that [function expressions](/en-US/docs/Web/JavaScript/Reference/Operators/function) are not hoisted:
```js
notHoisted(); // TypeError: notHoisted is not a function
var notHoisted = function () {
console.log("bar");
};
```
### Redeclarations
Whether `function` declarations can be redeclared in the same scope depends on what scope it's contained in.
At the top level of a script, `function` declarations behave like `var` and can be redeclared by another `function` or `var` but not by {{jsxref("Statements/let", "let")}}, {{jsxref("Statements/const", "const")}}, or {{jsxref("Statements/class", "class")}}.
```js-nolint example-bad
function a(b) {}
function a(b, c) {}
console.log(a.length); // 2
let a = 2; // SyntaxError: Identifier 'a' has already been declared
```
When `function` declarations are redeclared by `var`, the `var` declaration's initializer always overrides the function's value, regardless of their relative position. This is because function declarations are hoisted before any initializer gets evaluated, so the initializer comes later and overrides the value.
```js
var a = 1;
function a() {}
console.log(a); // 1
```
At the top level of a function's body, `function` also behaves like `var` and can be redeclared or have the same name as a parameter.
```js
function foo(a) {
function a() {}
console.log(typeof a);
}
foo(2); // Logs "function"
```
At the top level of a module or a block in strict mode, `function` declarations behave like `let` and cannot be redeclared by any other declaration.
```js-nolint example-bad
// Assuming current source is a module
function foo() {}
function foo() {} // SyntaxError: Identifier 'foo' has already been declared
```
```js-nolint example-bad
"use strict";
{
function foo() {}
function foo() {} // SyntaxError: Identifier 'foo' has already been declared
}
```
A `function` declaration within a `catch` block cannot have the same name as the `catch`-bound identifier, even in non-strict mode.
```js-nolint example-bad
try {
} catch (e) {
function e() {} // SyntaxError: Identifier 'e' has already been declared
}
```
## Examples
### Using function
The following code declares a function that returns the total amount of sales, when given the number of units sold of three products.
```js
function calcSales(unitsA, unitsB, unitsC) {
return unitsA * 79 + unitsB * 129 + unitsC * 699;
}
```
## Specifications
{{Specifications}}
## Browser compatibility
{{Compat}}
## See also
- [Functions](/en-US/docs/Web/JavaScript/Guide/Functions) guide
- [Functions](/en-US/docs/Web/JavaScript/Reference/Functions)
- {{jsxref("Function")}}
- [`function` expression](/en-US/docs/Web/JavaScript/Reference/Operators/function)
- {{jsxref("Statements/function*", "function*")}}
- {{jsxref("Statements/async_function", "async function")}}
- {{jsxref("Statements/async_function*", "async function*")}}
|