function declaration
The function declaration (function statement) defines a function with the specified parameters.
You can also define functions using the Function
constructor and a
function expression.
Syntax
function name([param[, param,[..., param]]]) {
[statements]
}
name
-
The function name.
param
Optional-
The name of an argument to be passed to the function. Maximum number of arguments varies in different engines.
statements
Optional-
The statements which comprise the body of the function.
Description
A function created with a function declaration is a Function
object and
has all the properties, methods and behavior of Function
objects. See
Function
for detailed information on functions.
A function can also be created using an expression (see function expression).
By default, functions return undefined
. To return any other value, the
function must have a return
statement that specifies
the value to return.
Conditionally created functions
Functions can be conditionally declared, that is, a function statement can be nested
within an if
statement, however the results are inconsistent across
implementations and therefore this pattern should not be used in production code. For
conditional function creation, use function expressions instead.
var hoisted = "foo" in this;
console.log(`'foo' name ${hoisted ? "is" : "is not"} hoisted. typeof foo is ${typeof foo}`);
if (false) {
function foo(){ return 1; }
}
// In Chrome:
// 'foo' name is hoisted. typeof foo is undefined
//
// In Firefox:
// 'foo' name is hoisted. typeof foo is undefined
//
// In Edge:
// 'foo' name is not hoisted. typeof foo is undefined
//
// In Safari:
// 'foo' name is hoisted. typeof foo is function
The results are exactly the same for a condition that evaluates to true
var hoisted = "foo" in this;
console.log(`'foo' name ${hoisted ? "is" : "is not"} hoisted. typeof foo is ${typeof foo}`);
if (true) {
function foo(){ return 1; }
}
// In Chrome:
// 'foo' name is hoisted. typeof foo is undefined
//
// In Firefox:
// 'foo' name is hoisted. typeof foo is undefined
//
// In Edge:
// 'foo' name is not hoisted. typeof foo is undefined
//
// In Safari:
// 'foo' name is hoisted. typeof foo is function
Function declaration hoisting
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');
};
Examples
Using function
The following code declares a function that returns the total amount of sales, when
given the number of units sold of products a
, b
, and
c
.
function calc_sales(units_a, units_b, units_c) {
return units_a * 79 + units_b * 129 + units_c * 699;
}
Specifications
Specification |
---|
ECMAScript Language Specification # sec-function-definitions |
Browser compatibility
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