在Javascript中使用===而不是==和typeof的原因是什么？ [英] What's the reason to use === instead of == with typeof in Javascript?

问题描述

在许多第三方库和最佳实践博客/推荐等中......通常会看到如下语法：

Throughout many third-party libraries and best practices blogs/recommendations, etc... it is common to see syntax like this:

typeof x === 'object' (instead of typeof x == 'object')
typeof y === 'string' (instead of typeof x == 'string')
typeof z === 'function' (instead of typeof x == 'function')

如果是typeof运算符已经返回一个字符串，还需要键入什么来检查返回值？如果typeof（typeof（x））总是 string ，无论x实际是什么，那么 == 就足够了 === 不必要的。

If the typeof operator already returns a string, what's the need to type check the return value as well? If typeof(typeof(x)) is always string, no matter what x actually is, then == should be sufficient and === unnecessary.

在什么情况下，typeof 不会返回字符串文字？即使有一些边缘情况，为什么附加类型检查用于对象，字符串，函数等...

Under what circumstances will typeof not return a string literal? And even if there's some fringe case why is the additional type check being used for object, string, function, etc...

推荐答案

回答主要问题 - 使用 == 使用 typeof 没有危险。以下是您可能想要使用 === 的原因。

To answer the main question - there is no danger in using typeof with ==. Below is the reason why you may want to use === anyway.

Crockford的建议是在许多情况下使用 === 会更安全，如果你打算在某些情况下使用它，那么最好是保持一致并将其用于所有事情。

The recommendation from Crockford is that it's safer to use === in many circumstances, and that if you're going to use it in some circumstances it's better to be consistent and use it for everything.

您可以考虑是否使用 == 或 === 每次检查平等，或者你可以养成总是写 === 。

The thinking is that you can either think about whether to use == or === every time you check for equality, or you can just get into the habit of always writing ===.

几乎没有理由使用 == 而不是 === - 如果您要比较 true 或 false 并且您想要强制（例如您想 0 或''评估为 false ）然后只需使用 if（！empty_str）而不是 if（empty_str == false）。

There's hardly ever a reason for using == over === - if you're comparing to true or false and you want coercion (for example you want 0 or '' to evaluate to false) then just use if(! empty_str) rather than if(empty_str == false).

对那些不了解问题的人 == 在typeof上下文之外，请参阅好的零件：

To those who don't understand the problems of == outside of the context of typeof, see this, from The Good Parts:

'' == '0'          // false
0 == ''            // true
0 == '0'           // true

false == 'false'   // false
false == '0'       // true

false == undefined // false
false == null      // false
null == undefined  // true

' \t\r\n ' == 0    // true

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