JavaScript检查变量是否存在（定义/初始化） [英] JavaScript check if variable exists (is defined/initialized)

问题描述

检查变量是否已初始化的哪种方法更好/更正？
（假设变量可以包含任何内容（字符串，整数，对象，函数等））

Which method of checking if a variable has been initialized is better/correct? (Assuming the variable could hold anything (string, int, object, function, etc.))

if (elem) { // or !elem

或

if (typeof(elem) !== 'undefined') {

或

if (elem != null) {

推荐答案

typeof 运算符将检查变量是否真的undefined。

The typeof operator will check if the variable is really undefined.

if (typeof variable === 'undefined') {
    // variable is undefined
}

typeof 运算符，与其他运算符在与未声明的变量一起使用时不会抛出 ReferenceError 异常。

The typeof operator, unlike the other operators, doesn't throw a ReferenceError exception when used with an undeclared variable.

但是，请注意 typeof null 将返回object。我们必须小心避免将变量初始化为 null 的错误。为了安全起见，这是我们可以使用的：

However, do note that typeof null will return "object". We have to be careful to avoid the mistake of initializing a variable to null. To be safe, this is what we could use instead:

if (typeof variable === 'undefined' || variable === null) {
    // variable is undefined or null
}

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有关使用严格比较的更多信息 === 而不是简单的相等 == ，请参阅：
在JavaScript比较中应使用哪个等于运算符（== vs ===）？

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For more info on using strict comparison === instead of simple equality ==, see:
Which equals operator (== vs ===) should be used in JavaScript comparisons?

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