如果有的话,Javascript的"typeof"会返回错误的类型吗？

记住:typeof是文字.

typeof undefined === "undefined"
typeof 5 === "number"
typeof true === "boolean"
typeof "" === "string"
typeof {} === "object"
typeof [] === "object"
typeof function { } === "function"

// null is a weird exception:
typeof null === "object"

instanceof区分什么东西是"函数"(注意对type "object"函数Object本身的字符串instanceof检查的typeof检查:

{} instanceof Object

function {} instanceof Object
function {} instanceof Function

[] instanceof Object
[] instanceof Array

如你所见,typeof说=== "function"和!== "object":这可能会产生误导,因为函数也是一个对象.这是instanceof发挥作用的时候.

现在,当您编写构造函数时,构造的对象也是您的函数的实例:

// the constructor
function Example { }

// the object
var ex = new Example();

typeof Example === "function"
typeof ex === "object"
ex instanceof Object
ex instanceof Example

使用原型你也可以扩展链,其中typeof总是说对象:

function DerivedExample { }

DerivedExample.prototype = new Example();
// DerivedExample.prototype.constructor = DerivedExample;

var ex1 = new DerivedExample();

typeof DerivedExample === "function"
typeof ex1 === "object"
ex1 instanceof Object
ex1 instanceof Example
ex1 instanceof DerivedExample

所以这是所有关于JavaScript typeof和也instanceof.希望它澄清正在发生的事情.

还有一件事要知道(建议不要在生产代码中使用,但我在CoffeeScript源代码中也看到过):

typeof new Number(5) === "object"
new Number(5) instanceof Object
new Number(5) instanceof Number

typeof new Boolean("true") === "object"
new Boolean("true") instanceof Object
new Boolean("true") instanceof Boolean

typeof new String("") === "object"
new String("") instanceof Object
new String("") instanceof String

关于函数和窗口对象:

每个独立函数都将在窗口的上下文中调用.
(除非)当你"use strict""指令"时,将使用undefined上下文调用独立函数.

function world() {
    // non-strict: this === window
    // use-strict: this === undefined
}
world();

现在,当您将函数编写为对象的成员时,函数的上下文是对象:

var hello = {
    world: function () {
        // this === hello
    }
};
hello.world();

对于在新构造的对象的上下文中调用的原型函数也是如此:

function Hello() { }

Hello.prototype.world = function () {
    // this instanceof Hello
    // this will be === ex
};

var ex = new Hello();
ex.world();

至少,您可以使用call和更改任何函数的上下文apply:

var ex = { };

function Example() {
    // (1) non-strict: this === window
    // (1) use-strict: this === undefined
    // (2) this === ex
}

Example(); // (1)
Example.call(ex);  // (2)
Example.apply(ex); // (2)