Why is this javascript code valid and works with comma inside brackets?

Question

var a = {
  key1: 1,
  key2: 2
};
console.log(a['key1', 'key2']); // print 2

Line 2 prints value 2, I don't understand why I do not have syntax error.

the good ol' `comma` operator - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator — Jaromanda X, Jun 09 '17 at 10:13
It is just a comma expression: https://developer.mozilla.org/it/docs/Web/JavaScript/Reference/Operators/Comma_Operator — Hitmands, Jun 09 '17 at 10:15

score 1 · Accepted Answer · answered Jun 09 '17 at 10:18

This code works just fine, because you are using Comma operator, so it will evaluate all the operands and return the last evaluated one.

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

1 Answers1