为什么 ("foo" === new String("foo")) 在 JavaScript 中评估为 false? [英] Why does ("foo" === new String("foo")) evaluate to false in JavaScript?

问题描述

我本打算在比较字符串值时一直使用 ===(三重等号，严格比较)，但现在我发现

I was going to start using === (triple equals, strict comparison) all the time when comparing string values, but now I find that

"foo" === new String("foo")

是假的，与此相同:

var f = "foo", g = new String("foo");
f === g; // false

当然:

f == g; // true

那么是建议字符串比较总是使用==，还是比较之前总是先把变量转成字符串?

So is it recommended to always use == for string comparison, or always convert variables to strings before comparing?

推荐答案

"foo" 是一个字符串 primitive.(这个概念在C#或Java中不存在)

"foo" is a string primitive. (this concept does not exist in C# or Java)

new String("foo") 是装箱的字符串对象.

new String("foo") is boxed string object.

=== 操作符 对原语和对象的行为不同.
比较原始类型(相同类型)时，如果它们具有相同的值，=== 将返回 true.

The === operator behaves differently on primitives and objects.
When comparing primitives (of the same type), === will return true if they both have the same value.

比较对象时，=== 仅当它们引用同一个对象(通过引用进行比较)时才会返回 true.因此，new String("a") !== new String("a").

When comparing objects, === will return true only if they refer to the same object (comparing by reference). Thus, new String("a") !== new String("a").

在您的情况下，=== 返回 false，因为操作数的类型不同(一个是原始类型，另一个是对象).

In your case, === returns false because the operands are of different types (one is a primitive and the other is an object).

原语根本就不是对象.
typeof 运算符不会为原语返回 "object".

Primitives are not objects at all.
The typeof operator will not return "object" for primitives.

当您尝试访问原始对象的属性(将其用作对象)时，Javascript 语言会将其装箱为一个对象，每次都创建一个新对象.这在规范中有所描述.

When you try to access a property of a primitive (using it as an object), the Javascript language will box it to an object, creating a new object every time. This is described in the specification.

这就是为什么你不能在基元上放置属性:

This is why you cannot put properties on primitives:

var x = "a";
x.property = 2;
alert(x.property) //undefined

每次编写 x.property 时，都会创建一个 不同的 装箱 String 对象.

Each time you write x.property, a different boxed String object is created.

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