Using object notation as if it were an array - why does this evaluate?

Question

I don't understand why the following evaluates to 3 instead of just declaring a syntax error when ran from a JavaScript REPL or through Chrome's Developer Tools:

{1, 2, 3};
3

As far as I can see, that should be a syntax error as demonstrated with:

var foo = {1, 2, 3};
Uncaught SyntaxError: Unexpected token ,

I feel like there's just some quirky behaviour I'm not aware of?

"I feel like there's just some quirky behaviour I'm not aware of?" -> that's proof of actual understanding of Javascript ! — Mathias Dolidon
– Mathias Dolidon, Commented Dec 14, 2016 at 15:24

Niet the Dark Absol · Accepted Answer · 2016-12-14 15:23:12Z

12

Here's the breakdown of the symbols:

{ Start code block
1 Number literal
, Comma operator (evaluates both sides, returns right side)
2 Number literal
, Comma operator
3 Number literal
} End code block

Code blocks aren't restricted to defining if, while etc. blocks, they can be used anywhere. Therefore, your code is simply a block that contains a chained comma operator sequence, which returns the last item in the chain, hence 3.

In the case of var foo = {1, 2, 3};, the { is indeed a "start object literal" symbol and not a "start code block" symbol.

The same symbol can have multiple meanings based on context.

answered Dec 14, 2016 at 15:23

Niet the Dark Absol

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1 Comment

Joe Shanahan Over a year ago

It hit me when I got to your first bullet point, thanks :) Will accept once SO deems the correct amount of time has passed.

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