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function - JavaScript | MDN

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The function declaration creates a binding of a new function to a given name.

You can also define functions using the function expression.

function calcRectArea(width, height) {
  return width * height;
}

console.log(calcRectArea(5, 6));
// Expected output: 30

function name(param0) {
  statements
}
function name(param0, param1) {
  statements
}
function name(param0, param1, /* …, */ paramN) {
  statements
}

name

The function name.

param Optional

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.

statements Optional

The statements which comprise the body of the function.

A function declaration creates a Function object. Each time when a function is called, it returns the value specified by the last executed return statement, or undefined if the end of the function body is reached. See functions for detailed information on functions.

function declarations behave like a mix of var and let:

• Like let, in strict mode, function declarations are scoped to the most closely containing block.
• Like let, function declarations at the top level of a module or within blocks in strict mode cannot be redeclared by any other declaration.
• Like var, function declarations at the top level of a script (strict or non-strict) become properties on 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 together with its value and can be called anywhere in its scope.

Warning: In non-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 statement. However, in non-strict mode, the results are inconsistent across implementations.

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.

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, block-level function declarations are scoped to that block and are hoisted to the top of the block.

"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

Function declarations in JavaScript are hoisted to the top of the enclosing function or global scope. You can use the function before you declared it:

hoisted(); // Logs "foo"

function hoisted() {
  console.log("foo");
}

Note that function expressions are not hoisted:

notHoisted(); // TypeError: notHoisted is not a function

var notHoisted = function () {
  console.log("bar");
};

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 let, const, or class.

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.

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.

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.

// Assuming current source is a module
function foo() {}
function foo() {} // SyntaxError: Identifier 'foo' has already been declared

"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.

try {
} catch (e) {
  function e() {} // SyntaxError: Identifier 'e' has already been declared
}

The following code declares a function that returns the total amount of sales, when given the number of units sold of three products.

function calcSales(unitsA, unitsB, unitsC) {
  return unitsA * 79 + unitsB * 129 + unitsC * 699;
}

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