定义var时未定义 [英] undefined thrown when defining var

问题描述

下一个代码在Google-chrome控制台中进行解释：

The next code is interpreted in Google-chrome console:

a = 123
123
     % Ok!

var b = 123
undefined
     % `undefined`? why? b is not undefined, it contains `123`.

推荐答案

它显示了操作的结果。

赋值表达式的结果是赋值的值。但是 var 语句不会产生结果，这不仅仅是因为它们不会出现在通常的代码流程中。我们可以在spec中的 var 语句的定义中看到：§13.3.2.4告诉我们 var 是抽象规范调用 NormalCompletion（空），它是只是的完整别名完成{[[type]]：normal，[[value]]：argument，[[target]] ：empty} 这是指出如何完成一个陈述/表达。在这种情况下，它完成没有价值。 （所有的JavaScript语句都不是这样，令人惊讶的是。）

The result of an assignment expression is the value that was assigned. But var statements don't have a result, not least because they don't occur within the usual step-by-step flow of code. We can see this in the definition of the var statement in the spec: §13.3.2.4 tells us that the result of var is the abstract specification call NormalCompletion(empty), which is just an alias for Completion{[[type]]: normal, [[value]]: argument, [[target]]:empty} which is for indicating how a statement/expression completed. In this case, it completes with no value. (Which isn't true of all JavaScript statements, surprisingly.)

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