JavaScript：测试未定义值的变量;测试对象是否是数组 [英] JavaScript: Testing variables for undefined value; testing whether object is array

问题描述

1. typeof value ===undefined和值是否有区别=== undefined ？

为什么JavaScript需要 Array.isArray（） in ECMAScript 5？我不能只调用 value instanceof Array 来确定给定变量是否是一个数组？

Why did JavaScript need Array.isArray() in ECMAScript 5? Can't I just call value instanceof Array to determine whether an given variable is an array?

推荐答案

//var value; There is no var declaration. The variable was never declared

// check againts undeclared variables
typeof value === "undefined"; // works

// check againts declared variables with no value
value === undefined; // ReferenceError: value is not defined

还存在 undefined =的问题true 有效。但你并不真正关心这一点。这些天没有人愚蠢到足以改变 undefined 。

There are also issues with undefined = true being valid. But you don't really care about that. No-one is stupid enough to alter undefined globally these days.

我也知道<$ c的错误$ c> instanceof 被破坏了。我不能告诉你为什么 Array.isArray 更好的原因。

I also know of bugs with instanceof being broken. I can't give you the exact reason why Array.isArray is better though.

你会发现批评 JavaScript Garden 中的 instanceof

如果你阅读这篇文章它提到了 instanceof 在单独的框架/窗口/ iframe中是如何工作的。

If you read this Article It mentions how instanceof does not work across seperate frames / windows / iframes.

因为instanceof检查了code>数组，每个窗口都有自己的 window.Array 。

Because instanceof checks againts Array and each window has it's own window.Array.

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