Web/JavaScript/Reference/Operators/Comma operator-2

The comma operator (,) evaluates each of its operands (from left to right) and returns the value of the last operand. This lets you create a compound expression in which multiple expressions are evaluated, with the compound expression's final value being the value of the rightmost of its member expressions. This is commonly used to provide multiple parameters to a for loop.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

expr1, expr2, expr3...

Parameters

expr1, expr2, expr3...: One or more expressions, the last of which is returned as the value of the compound expression.

Usage notes

You can use the comma operator when you want to include multiple expressions in a location that requires a single expression. The most common usage of this operator is to supply multiple parameters in a for loop.

The comma operator is fully different from the comma within arrays, objects, and function arguments and parameters.

Examples

If a is a 2-dimensional array with 10 elements on each side, the following code uses the comma operator to increment i and decrement j at once.

The following code prints the values of the diagonal elements in the array:

for (var i = 0, j = 9; i <= 9; i++, j--)
  console.log('a[' + i + '][' + j + '] = ' + a[i][j]);

Note that the comma operators in assignments may appear not to have the normal effect of comma operators because they don't exist within an expression. In the following example, a is set to the value of b = 3 (which is 3), but the c = 4 expression still evaluates and its result returned to console (i.e., 4). This is due to operator precedence and associativity.

var a, b, c;

a = b = 3, c = 4; // Returns 4 in console
console.log(a); // 3 (left-most)

var x, y, z;

x = (y = 5, z = 6); // Returns 6 in console
console.log(x); // 6 (right-most)

Processing and then returning

Another example that one could make with comma operator is processing before returning. As stated, only the last element will be returned but all others are going to be evaluated as well. So, one could do:

function myFunc() {
  var x = 0;

  return (x += 1, x); // the same as return ++x;
}

Specifications

Specification

ECMAScript (ECMA-262)The definition of 'Comma operator' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
Comma operator	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 3	Opera Full support 4	Safari Full support 1	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 10.1	Safari iOS Full support 1	Samsung Internet Android Full support 1.0	nodejs Full support 0.1.100

Legend

Full support: Full support