javascript - 功能之前，感叹号有什么作用？(What does the exclamation mark do before the function?)

Question

!function () {}();

  ask by Sebastian Otto translate from so

Answer 1

JavaScript syntax 101. Here is a function declaration :

(JavaScript语法101。这是一个函数声明 ：)

function foo() {}

Note that there's no semicolon: this is just a function declaration .

(请注意，这里没有分号：这只是一个函数声明 。)

You would need an invocation, foo() , to actually run the function.

(您需要一个调用foo()才能实际运行该函数。)

Now, when we add the seemingly innocuous exclamation mark: !function foo() {} it turns it into an expression .

(现在，当我们添加看似无害的感叹号： !function foo() {} ，它将变成一个表达式 。)

It is now a function expression .

(现在它是一个函数表达式 。)

The !

(!)

alone doesn't invoke the function, of course, but we can now put () at the end: !function foo() {}() which has higher precedence than !

(当然，仅单独调用不会调用该函数，但是我们现在可以将()放在最后： !function foo() {}()优先级高于!)

and instantly calls the function.

(并立即调用该函数。)

So what the author is doing is saving a byte per function expression;

(因此，作者正在做的就是为每个函数表达式保存一个字节。)

a more readable way of writing it would be this:

(更具可读性的书写方式是：)

(function(){})();

Lastly, !

(最后， !)

makes the expression return true.

(使表达式返回true。)

This is because by default all IIFE return undefined , which leaves us with !undefined which is true .

(这是因为默认情况下，所有IIFE返回的都是undefined ，这给我们留下!undefined ，它为true 。)

Not particularly useful.

(不是特别有用。)