Why the result is 8?

Question

Today I had a job interview on JS and one of the questions was

What is the value of a

var a = (3,5 - 1) * 2;

Hmm I assumed its 8 (the chrome dev console also gives it 8) but I don't know why it is eight. Why the 3 is omitted? I mean of course you can't do any operations with it but still, it disappears, or I am wrong?

Please go easy on me. I am trying to understand. Any articles on this kind of operations would be highly appreciated

See this blog post. https://javascriptweblog.wordpress.com/2011/04/04/the-javascript-comma-operator/ — Jonathan, Apr 20 '17 at 17:21
That's not a `.` for floating point. That's a comma operator (the kind you see in for loops) which groups statements in one statment and evaluating to the result of the last. — ibrahim mahrir, Apr 20 '17 at 17:22

score 2 · Accepted Answer · answered Apr 20 '17 at 17:22

2

As per the Comma Operator | MDN,

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

So, var a = (3,5 - 1) * 2; returns 4 * 2 = 8

answered Apr 20 '17 at 17:22

tanmay

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`(3,5-1)` evaluates to `4` – Jonathan Apr 20 '17 at 17:23
_...the value of the last (left-most) operand_, he said! – ibrahim mahrir Apr 20 '17 at 17:26
@ibrahimmahrir yes, it was a mistake. thanks for pointing out – tanmay Apr 20 '17 at 17:29
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Yeah I replaced the downvote with an upvote. The other downvote is probably going to stay there as the downvoter is probably left ages ago. – ibrahim mahrir Apr 20 '17 at 17:32

Marcos Casagrande · Answer 2 · 2017-04-20T17:24:56.263

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3,5 it's not 3.5 (float). See comma operator

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

3,5 equals 5.

(3,5 - 1) * 2 translates to (5 - 1) * 2 which is 8.

edited Apr 20 '17 at 17:24

answered Apr 20 '17 at 17:22

Marcos Casagrande

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I know its not a float but now I will know about comma operator :D – Nicholas Apr 20 '17 at 17:24
Check the link I provided! – Marcos Casagrande Apr 20 '17 at 17:25

score 1 · Answer 3 · answered Apr 20 '17 at 17:22

1

See this reference:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator

The comma operator evaluates all of the expressions listed and returns the last one. So your expression simplifies to

(5 - 1) * 2

answered Apr 20 '17 at 17:22

n8wrl

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Why the result is 8?

3 Answers3