在javascript中访问其他对象的属性

Question

在属于javascript对象的JavaScript函数中，我想使用另一个对象的属性值：

var firstObject={
    says:"something"
}

var secondObject={
    speak:function(){console.log(this.saysToo)},
    saysToo:firstObject.says
}

secondObject.speak(); 

在调试器中检查“ secondObject”时，“ saysToo”具有正确的值。但是，如果我尝试通过“ this.saysToo”访问它，则它是未定义的。

如何从第二个对象中访问第一个对象的属性？

Answer 1

这两个firstObject和secondObject是单独的对象。关键字this是指其执行上下文的对象。

<script>
var firstObject = {
    says: "something",
    test: function() {
        //this == firstObject
        console.log(this == firstObject); //shows: true
    }
}

var secondObject = {
    speak: function() {
        //this == secondObject
        console.log(this.saysToo);
    },
    saysToo: firstObject.says,
    test: function() {
        //this == secondObject
        console.log(this == secondObject); //shows: true
        console.log(this == firstObject); //shows: false
    },
}

secondObject.speak();

//this == window
console.log(this===window); //shows: true

console.log(typeof this.saysToo); //shows: undefined
//because "this.saysToo" is same as "window.saysToo" in this (global) context
</script>

可以使用callof apply方法将一个函数调用与其他对象绑定，以使this该函数充当另一个对象。

<script>
var firstObject = {
    says: "something",
    saysToo: "other"
}

var secondObject = {
    speak: function() {
        console.log(this.saysToo);
    },
    saysToo: firstObject.says
}

secondObject.speak(); //shows: "something"

//bind with "firstObject"
secondObject.speak.call(firstObject); //shows: "other"
</script>