javascript - JS : Why does function declaration order matter when inside a conditional block?

Question

a();
function a() {
    $('.doit').text('Text was replaced (a)');   
}

if ($('.doit2').length) {
    b();
    function b() {
         $('.doit2').text('Text was replaced (b)');   
    }
}

a() 被正确调用，而 b 给出错误 “b 未定义”。为什么？

(我读过有关提升的内容，但声明了function b，而不是变量。还是我错了？)

参见fiddle - Firefox 抛出错误，而 Chrome 正常。

Answer 1

根据 Javascript 规范，条件语句(或任何其他 block )中不允许使用函数声明。因此，这在技术上是未定义的行为。一些浏览器尝试创建合理的行为。但是，您不应该依赖浏览器能够正确解释这一点。

FunctionDeclarations are only allowed to appear in Program or FunctionBody. Syntactically, they can not appear in Block ({ ... }) — such as that of if, while or for statements.

http://kangax.github.io/nfe/#function-declarations-in-blocks