为什么控制台上的JavaScript变量声明导致“未定义”？正在印刷？ [英] Why does JavaScript variable declaration at console results in "undefined" being printed?

问题描述

我已阅读以下SO帖子：

I have already read the following SO posts:

为什么这个JavaScript代码在控制台上打印未定义？

为什么Chrome& FireFox控制台打印未定义？

为什么JS控制台会返回一个额外的未定义？

但这些都没有解释为什么JavaScript控制台打印当我按如下方式声明变量时未定义：

But none of it explains why the JavaScript console prints undefined when I declare a variable as follows:

var a;

推荐答案

它打印此表达式的结果 - undefined 。是的， var a 本身就是一个有效的表达式。

It prints the result of this expression - which is undefined. And yes, var a is a valid expression on its own.

实际上，你应该选择当你写 var a = 3 console 打印 undefined 会很开心$ c>或类似的东西。如果处理函数anyFunctionName（）{} 语句，它还会打印 undefined 。事实上，如果有另一个声明，那么所有 var 和函数声明（！）语句似乎都会被忽略真实的结果：

Actually, you should rather be amused by why console prints undefined when you write var a = 3 or something like this. It also prints undefined if function anyFunctionName() {} statement is processed. In fact, all the var and function declaration (!) statements seem to be ignored if there's another statement with some 'real' result:

>>> var a = 3;
undefined

>>> var a = 3; a = 4;
4

>>> var a = 3; a = 4; var a = 5; function f() {};
4 // !!!

现在，我认为背后的真正原因是 eval的行为声明，如 此处 ：

Now, I suppose the real reason behind is behaviour of eval statement, as described here:

• 让结果成为评估程序 prog 。


• 如果 result.type 是正常，其完成值为值V ，然后返回值V 。


• 如果 result.type 是正常，其完成值为清空，然后返回值 undefined 。

• Let result be the result of evaluating the program prog.
• If result.type is normal and its completion value is a value V, then return the value V.
• If result.type is normal and its completion value is empty, then return the value undefined.

现在的问题是， var a = 4 语句返回是什么？猜猜看：它不是4。

So now the question is, what does var a = 4 statement return? Guess what: it's not 4.

生产 VariableStatement ： var VariableDeclarationList; 是
评估如下：

The production VariableStatement : var VariableDeclarationList; is evaluated as follows:

• 评估VariableDeclarationList。


• 返回（正常，空，空））。

现在最有趣的部分：上一个例子中发生了什么，为什么4结果呢？这在此部分中有解释：

Now the most interesting part: what happened in the last example, why 4 is the result? That's explained in this section:

生产程序： SourceElements 的计算方法如下：

• 让结果成为评估SourceElements的结果。

[...]

生产 SourceElements ： SourceElements * SourceElement *评估如下：

The production SourceElements : SourceElements *SourceElement* is evaluated as follows:

• 让headResult成为评估SourceElements的结果。


• 如果headResult突然完成，则返回headResult。


• 让tailResult是评估SourceElement的结果。


• 如果tailResult.value为空，则让V = headResult.value，否则让V => tailResult.value。


• 返回（tailResult.type，V，tailResult.target）

• Let headResult be the result of evaluating SourceElements.
• If headResult is an abrupt completion, return headResult.
• Let tailResult be result of evaluating SourceElement.
• If tailResult.value is empty, let V = headResult.value, otherwise let V = > tailResult.value.
• Return (tailResult.type, V, tailResult.target)

两个函数f（）{} 和 var a = 5 语句'返回值（正常，空，空）。所以脚本最终给出了第一个语句的结果（从脚本的结尾开始，技术上它是最后一个），它不是（正常，空，空） 。这是 a = 4 赋值语句的结果 - 4 。

Both function f() {} and var a = 5 statements' return values were (normal, empty, empty). So the script ended up with giving out the result of the first statement (starting from the script's end, so technically it's the last one) that's not (normal, empty, empty). That is the result of a = 4 assignment statement - which is 4.

PS现在有些锦上添花：请考虑以下因素：

P.S. And now for some icing on the cake: consider the following:

>>> function f() {}
undefined

>>> (function f() {})
function f() {}

区别在于非常微妙：第一个输入被视为函数声明语句，根据此规则......

The difference is quite subtle: the first input is treated as a Function Declaration statement, which, according to this rule...

生产 SourceElement ： FunctionDeclaration 评估为
如下：

The production SourceElement : FunctionDeclaration is evaluated as follows:

• 返回（正常，空，空）。

..我们已经知道，当 eval -ed时，最终会产生 undefined 。

... will eventually produce undefined when eval-ed, as we already know.

然而，第二个输入被视为函数表达式，其被评估到函数本身。这意味着它将通过 eval 传递，并最终返回到控制台（格式）。

The second input, however, is treated as a Function Expression, which is evaluated to the function itself. That means it'll be passed through eval and eventually returned to the console (in its format).

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