JavaScript 函数表达式前的加号 [英] JavaScript plus sign in front of function expression

问题描述

我一直在寻找有关立即调用函数的信息，并且在某处偶然发现了这个符号:

I’ve been looking for information about immediately invoked functions, and somewhere I stumbled on this notation:

+function(){console.log("Something.")}()

有人可以向我解释一下函数前面的 + 符号是什么意思吗?

Can someone explain to me what the + sign in front of the function means/does?

推荐答案

它强制解析器将 + 后面的部分视为表达式.这通常用于立即调用的函数，例如:

It forces the parser to treat the part following the + as an expression. This is usually used for functions that are invoked immediately, e.g.:

+function() { console.log("Foo!"); }();

如果没有 + ，如果解析器处于期望语句(可以是表达式或多个非表达式语句)的状态，则单词 function 看起来像一个函数声明的开始，而不是一个函数表达式，所以它后面的()(在上面的行)将是一个语法错误(在该示例中，名称的缺失也是如此).使用 +，它使它成为一个函数表达式，这意味着名称是可选的，并且导致对函数的引用，该函数可以被调用，因此括号是有效.

Without the + there, if the parser is in a state where it's expecting a statement (which can be an expression or several non-expression statements), the word function looks like the beginning of a function declaration rather than a function expression and so the () following it (the ones at the end of the line above) would be a syntax error (as would the absense of a name, in that example). With the +, it makes it a function expression, which means the name is optional and which results in a reference to the function, which can be invoked, so the parentheses are valid.

+ 只是选项之一.它也可以是 -、!、~ 或几乎任何其他一元运算符.或者，您可以使用括号(这更常见，但在语法上既不正确也不正确):

+ is just one of the options. It can also be -, !, ~, or just about any other unary operator. Alternately, you can use parentheses (this is more common, but neither more nor less correct syntactically):

(function() { console.log("Foo!"); })();
// or
(function() { console.log("Foo!"); }());

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