为什么在 JavaScript 类 A 中的 instanceof 函数，而 typeof 类 A 不是对象? [英] Why in JavaScript class A instanceof Function, but typeof class A is not an object?

问题描述

当我们说instance of"时，我们假设我们正在处理一个对象.为什么 JavaScript 的操作符 instanceof 在我们询问 (class A { }) instanceof Function 时返回 true，而 typeof (class A { })==函数"?为什么不是object?

When we say "instance of", we assume that we are dealing with an object. Why JavaScript's operator instanceof returns true when we ask (class A { }) instanceof Function, but typeof (class A { }) == "function"? Why not object?

推荐答案

当我们询问(class A { }) instanceof Function

classes 只是构造函数的语法糖.IE.class A {} 的求值产生一个函数.

classes are just syntactic sugar for constructor functions. I.e. the evaluation of class A {} produces a function.

以下两个示例是(或多或少)等价的，即它们产生相同的结果/值:

The following two examples are (more or less) equivalent, i.e. they produce the same result/value:

// class
class A {
  constructor() {
    this.foo = 42;
  }

  bar() {
    console.log(this.foo);
  }
}

// constructor function
function A() {
  this.foo = 42;
}

A.prototype.bar = function() {
  console.log(this.foo);
}

所有非原始值(字符串、数字、布尔值、空值、未定义、符号)在 JavaScript 中都是对象.函数也是对象，具有特殊的内部属性，使它们可调用(和/或可构造).

Everything that is not a primitive value (string, number, boolean, null, undefined, symbol) is an object in JavaScript. Functions are objects too, with special internal properties that makes them callable (and/or constructable).

为什么不反对?

typeof 返回字符串 "function" 作为函数值，因为它是在规范中定义的.

typeof returns the string "function" for function values because that's how it is defined in the specification.

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