理解Javascript Typeof [英] understanding Javascript Typeof

问题描述

当我执行下面的代码时，它会打印两次未定义".我原以为它会引发错误，因为未定义变量，并且顶部还有 use strict' 语句.

When I execute below code , it prints "undefined" two times. I was expecting that it would raise error since variable is not defined and also there is use strict' statement in the top.

'use strict';
var a;

console.log(typeof a);
console.log(typeof b);

谁能解释为什么它没有引发错误?

Can anyone explain why it is not raising an error ?

推荐答案

实际上在 JavaScript 中 undefined 意味着变量尚未定义，所以基本上:

In fact in JavaScript undefined means that the variable isn't yet defined, so basically :

• typeof a 返回 undefined 因为变量 a 仅声明但尚未初始化(没有分配给它的值).

• typeof a returns undefined because the variable a was only declared but not initialized yet (there's no value assigned to it).

并且 typeof b 返回 undefined 因为变量 b 尚未声明，因此未定义.

And typeof b returns undefined because the variable b is not yet declared, so isn't defined.

如果变量没有赋值，它的类型为 undefined 因为无法确定类型.

And if there's no value assigned to a variable, it gets the type undefined because as type can't be determined.

因此，如果您查看 MDNtypeof 规范你会看到:

So if you check the MDN typeof specification you will see that :

typeof 运算符返回一个字符串，指示未计算的操作数的类型，如果您看到类型表，您可以看到 undefined 是一个原始类型，并且是typeof 的可能返回值.

The typeof operator returns a string indicating the type of the unevaluated operand, and if you see types table you can see that undefined is a primitive type and one of the possible return values of typeof.

示例:

您可以在示例部分看到未定义的返回:

And you can see in the Examples section, the undefined return:

//未定义

typeof undefined === 'undefined';

typeof undefined === 'undefined';

typeof DeclarationButUndefinedVariable === 'undefined';

typeof declaredButUndefinedVariable === 'undefined';

typeof undeclaredVariable === 'undefined';

typeof undeclaredVariable === 'undefined';

注意:

正如评论中所述，这仅与 JavaScript 语法有关，与 nodejs 无关.

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