The purpose of the comma operator in JavaScript (x, x1, x2, …, xn) - Stack Overflow

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In JavaScript (x, x1, x2, …, xn) always returns xn.

In Python this code is called tuple and it returns different values.

My question is what is the purpose of this code in JavaScript? Do you have some practical example?

In JavaScript (x, x1, x2, …, xn) always returns xn.

In Python this code is called tuple and it returns different values.

My question is what is the purpose of this code in JavaScript? Do you have some practical example?

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Share Improve this question Follow edited Sep 9, 2015 at 7:49 vaultah 46.6k1212 gold badges119119 silver badges144144 bronze badges asked Aug 3, 2011 at 14:00 antonjsantonjs 14.3k1515 gold badges6969 silver badges9191 bronze badges

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3 Answers 3

Sorted by: Reset to default 10

The ma operator evaluates every operand but only returns the result of the last evaluation. It can be used to initialize multiple variable in a for loop.

var str = "Hello, World!";
for(var i = 0, len = str.length; i < len; i++)
    console.log(str.charAt(i));

Edit: As pointed out in the ments, this isn't a true example of the ma operator, this is just the var. You can see this page for actual examples.

http://en.wikipedia/wiki/Comma_operator

Like in C and C++, the ma operator evaluates both the left and right operands, returning the right operand. In terms of practical uses, a, b can be used in place of a ? b : b, as well as used as a limited way to fit multiple 'statements' into one statement, such as in the first or third parts of a for loop.

Note that declaring and/or defining multiple variables with var, separated by mas, is not a use of the ma operator, even though it uses the ma symbol. The ma operator evaluates to and forms an expression, which can be used as a value as part of a larger expression or statement. var is a plete statement type that takes the form

var name1 [= value1][, name2 [= value2][, ...]];

The MDC docs has a good example of a practical use for the ma operator:

If a is a 2-dimensional array with 10 elements on a side, the following code uses the ma operator to increment two variables at once. The code prints the values of the diagonal elements in the array:

for (var i = 0, j = 9; i <= 9; i++, j--)
  document.writeln("a[" + i + "][" + j + "] = " + a[i][j]);

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