javascript - Comma operator returns first value instead of second in argument list? - Stack Overflow

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MDN claims that:

The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand.

However, when I tried running <script> alert(1, 2); </script>, it shows a "1" instead of a "2".

Am I misunderstanding something?

MDN claims that:

The comma operator evaluates both of its operands (from left to right) and returns the value of the second operand.

However, when I tried running <script> alert(1, 2); </script>, it shows a "1" instead of a "2".

Am I misunderstanding something?

• javascript
• operators
• comma-operator

Share Improve this question Follow edited Jan 28, 2016 at 1:45 user4639281 asked Apr 7, 2011 at 11:54 PacerierPacerier 89.6k111111 gold badges383383 silver badges644644 bronze badges

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3 Answers 3

Sorted by: Reset to default 19

In the context of a function call, the comma is used to separate parameters from each other. So what you're doing is passing a second parameter to alert() which gets silently ignored.

What you want is possible this way:

 alert((1,2));

The extra brackets form a parameter on their own; inside them you can use the comma as an operator.

Comma(,) is also a parameter separator.

Use alert((1,2)) instead.

When you use it like that, the comma is not an operator, it's a separator between the parameters in the call to the alert method.

If you put parentheses around them so that it's an expression, it will show you 2:

alert( (1,2) );

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