Javascript: rounded parentheses surrounding comma separated expressions
Playing on JS console I faced a curious syntax. I wonder if someone can tell me more on that..
Try this:
>( function f(){console.log('i am f')} , (function x(){console.log('i am x')})() , y=2 , console.log('hello') )
i am x
hello
undefined
>f()
ReferenceError: f is not defined
>this.y
2
This will fail:
( var c=2 ) SyntaxError: Unexpected token var
So comma separated expressions inside parentheses are evaluated, assignments happens to be against global scope, but named function declarations references stay trapped inside just like a closure more... putting that line inside a function declaration called with new:
function C(){
( function f(){console.log('i am f')} , (function x(){console.log('i am x')})() , y=2 , console.log('hello') )
}
and then instantiating:
>var c=new C()
i am x
hello
undefined
>c.y
undefined
>this.y
2
Happens exactly the same, just as executed in the global scope!
What's the usage / purpose of this construct?
One more:
>( function f(){console.log('i am f')} , f() )
ReferenceError: f is not defined
So the named function can't be referenced neither inside brackets.
The named functions aren't "trapped inside", because they're not function declarations (using function as a statement), they are in fact function expressions (using function as an operator). This means that their names do not become references to themselves in the current namespace.
Certain keyword/tokens can only be used as statements, such as var , hence an error is thrown if you try to use them in conditions which the interpreter expects to be an expression.
As for the y === 2 , this is because you did not var y; inside C , so y = 2 sets window.y , and in the global scope this === window .
whats the usage / purpose of this construct?
, lets you do multiple expressions on one line. Wrapping code in parentheses forces it to be parsed as an expression.
The var keyword is only valid as a statement, so this creates a syntax error.
Function declarations can either be statements (which create a hoisted variable) or expressions (which do not create any variable).
Thus, wrapping the function in parentheses turns it into a function expression, which does not create an externally-visible name.
For more information, see here.
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