Why is this grouping operator + function immediately invoked

iife, javascript

Solution

Because you forgot the semicolon after the first function expression:

(function () {
    document.write("bar");
});

Otherwise the second "grouping operator" is interpreted as a function call. So this:

(function a() {
    ...
})

(function b() {
    ...
}());

is basically the same as:

function b() {
    ...
}

(function a() {
    ...
})(b());

Reordering makes it a bit easier to see. Remember that white space characters have no meaning in JavaScript and are ignored.

Problem

I'am studying the behaviour of Immediatly Invoked Function Expressions (IIFE) and while doing that I encounterd the following situation. ``` (function () { document.write("bar"); }) (function () { document.write("foo"); }()); ``` I thought that the first is just a grouping operator with a function expression inside without calling it. The second is a grouping operator as well with a function expression but now with the call of that function. What I find strange is that both are invoked, why is that? ``` (function () { document.write("bar"); }) var x = 1; (function () { document.write("foo"); }()); ``` When I break the two by inserting a variable declaration in between, it's just writes foo. This is what I expected.

Original source