Why do we put '(' before fucntion in JS?

Question

Why do we have ( before function word here?

(function() {

     var message = "Привет"; function showMessage() {
    alert( message );   }

     showMessage();

})();

to make it an inline function expression from function declaration so that you can immediately invoke it. — gurvinder372, Jan 23 '16 at 08:02
To enclose the function into it's own closure, all variables defined with in it will not have conflict with other global variables — Adam Azad, Jan 23 '16 at 08:02
@AdamAzad: That's what the function is for, not the `(` before it. — T.J. Crowder, Jan 23 '16 at 08:03
@AdamAzad: What about it? Unless you put a function expression where `/* code */` is, no scope is created, and that will fail with an error. — T.J. Crowder, Jan 23 '16 at 08:06
Re closing as duplicate: Although the earlier question asks about `+`, the answers there answer this question (which is the dupe test on SO). — T.J. Crowder, Jan 23 '16 at 08:06
@Derek朕會功夫 And in ES5 and earlier, only *functions* do. — T.J. Crowder, Jan 23 '16 at 08:33

score 1 · Answer 1 · answered Jan 23 '16 at 08:04

Try not to:

function() { return 1; }()

then you will get Uncaught SyntaxError: Unexpected token (

JavaScript parser runs in two modes, lets call it expression mode and normal mode, in normal mode JS parser expects top level declarations like functions and code blocks. You use '(' to enter expression mode, in expression mode function() { } will be interpreted as constant whose value is a function.

There is similar case with objects literals:

{ foo: 1 }

without '(' this means block of code, where you have single expression - constant 1 proceeded by label, when you use ({ foo: 1 }) parser enters expression mode and interprets it as object literal with property foo.

Why two modes, it is enforced by language grammar which in case of JS is pretty complicated (like in most C based languages).

Why do we put '(' before fucntion in JS?

1 Answers1