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?

Answer 1

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.