“Uncaught TypeError：undefined不是函数”在JavaScript代码块中 [英] "Uncaught TypeError: undefined is not a function" in JavaScript code block

问题描述

我的html页面中有一个如下所示的JavaScript代码块。当我通过加载页面来运行它。我的浏览器控制台输出低于输出。

I have a JavaScript code block like below in my html page. When I run this by loading the page. I am getting below output on my browser console.

outer world 
Uncaught TypeError: undefined is not a function

正如您在代码片段中看到的，我没有执行名为 b的函数代码中的任何位置。但是在运行代码时，该函数的输出随附 undefined不是函数错误我找不到我的代码块中的任何位置。

As you can see in the code snippet, I am not executing the function named b anywhere in the code. But on running the code, the output from that function is coming along with an undefined is not a function error which I could not locate anywhere in my code block.

要在此方案中添加更多内容，当我删除代码中的任何一个部分时，没有日志。那是。如果我从代码中删除b的初始化，则没有错误和输出。此外，如果我删除自执行功能块，则没有日志或错误。确实b的初始化行缺少分号。但诱惑它提供这样的输出会让我感到困惑。你能帮助我找出这种行为的推理吗？

To add more to this scenario, there is no logs when I remove any one of the parts in the code. that is. If I remove b's initialization from the code then there are no errors and output. Also if I remove the self executing function block, there are no logs or errors. Its true that the b's initialization line is missing a semicolon. but what tempts it to provide such an output confuses me. Would you help me out to figure out a reasoning for this behaviour?

你能帮我理解为什么会这样吗？

Can you please help me understand why it is happening so?

var b = function() {
  console.log('outer world');
}

(function() { 

})();

推荐答案

在声明 b 后错过了; 。以下代码与您拥有的代码相同。

Missed a ; after declaring b. The following code is equivalent to what you have.

var b = function() {
   console.log ('outer world');
}(function() {})();

没有; b变为自动执行并将一个空函数作为参数。之后它再次自我执行;但是，因为b没有返回函数，你会得到一个例外。

Without the ; b becomes self-executing and takes an empty function as a parameter. After that it self-executes again; however, because b does not return a function, you get an exception.

我建议不要跳过; 直到你成为js忍者:)。保留 b 以避免全球范围内的污染。

I suggest not to skip ; until you become js ninja :). Keep b inside to avoid global scope pollution.

(function () {
    "use strict";

    var b = function () {
        console.log("hi");
    };
    // other code
}());

分号自动插入讨论

如果你不想要分号，添加自运行函数之前的运算符

In case you do not want semicolons, add an operator before self-running function

var b = function () { console.log('outer world') }
;(function() { /* ... */ }())

ES6更新：

(() => {
  const b = () => console.log('hi')
  // ...
})()

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