为什么立即调用此分组运算符+函数

问题描述

我正在研究Immediatly Invoked Function Expressions（IIFE）的行为，在这样做时我遇到了以下情况。

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");
}());

我认为第一个只是一个分组运算符，里面有一个函数表达式而没有调用它。第二个是分组运算符以及函数表达式，但现在调用该函数。

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.

我觉得奇怪的是两者都被调用了，为什么会这样？

What I find strange is that both are invoked, why is that?

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

var x = 1;

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

当我通过在两者之间插入变量声明来打破两者时，它只是写入foo。这就是我的预期。

When I break the two by inserting a variable declaration in between, it's just writes foo. This is what I expected.

推荐答案

因为你忘记了第一个函数表达式后面的分号：

Because you forgot the semicolon after the first function expression:

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

否则第二个分组运算符被解释为函数调用。所以这个：

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

(function a() {
    ...
})

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

基本相同：

function b() {
    ...
}

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

重新排序让它更容易看到。请记住，空白字符在JavaScript中没有意义，会被忽​​略。

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

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