JavaScript 题 库 (三)

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JavaScript 题库(对象 & this)

1. 对象的键名只能是字符串, 如果不是字符串需要先通过toString() 方法转化成字符串

var a = {},
    b = {
        key: 'b'
    },
    c = {
        key: 'c'
    };
a[b] = 123;
a[c] = 456;
/*
            a = {
                [object Object]: 456
            }
        */
console.log(a[b]); // 456

2. this 默认指向window, 函数调用时, this指向调用函数的对象

var name = 'python'

function fn() {
    console.log(this.name)
}
var obj = {
    name: 'java',
    sayName: fn,
    say: function() {
        console.log(this.name);
        var x = fn;
        x(); //AO{this:window} //python
    }
}
obj.sayName(); // AO{this:window}  //java
obj.say(); // AO{this:obj, x:undeined}  //java
//GO{name: python, fn:fn, obj:{}}

3.

var n = 10;
var obj = {
    n: 5,
    c: a()
};

function a() { //AO{this:obj, n:undefined, }
    var n = 7;
    var c = function() {
        var n = 3;
        console.log(this.n); //5
    }
    return c;
}
obj.c(); //a()()
// GO{n:10, obj:{}, a:fn}

4.

var obj = {
    foo: 'bar',
    fun: function() { //AO{this:obj, self:obj}
        var self = this;
        console.log(this.foo); // 'bar'
        console.log(self.foo); // 'bar'
        (function() {
            console.log(this.foo); //'undefined'
            console.log(self.foo) // 'bar' 作用域链查找
        })() //AO{this:window}
    }
}
obj.fun();

5.

// 没有通过var声明的变量都是全局的; typeof aaaaa == 'undefined'
(function() {
    var a = b = 3; // ==> b = 3; var a = b;
})();
console.log(typeof a !== 'undefined'); // typeof a == 'undefined'  // false 
console.log(typeof b !== 'undefined'); // true

6.

var result = false === 1;
console.log(result); //false
if (typeof c && -true + (+undefined) + '') { 
    // 'undefined' && -true + NaN + '' ==> // 'undefined' && 'NaN' ==> // 找假
    console.log('python'); // 'python'

}
if (22 + '33' * 2 == 88) {
    console.log('java'); // 'java'

};
!!' ' + !!'' - !!false || console.log('javascript'); // 1
// 1 + 0 - 0 || 'javascript' ==> 1 || 'javascript'

7.

var a = 1,
    b = c = 0;

function add(n) {
    return n = n + 3;
}
y = add(a); // 5

function add(n) {
    return n = n * 5;
}
z = add(a); //5
console.log(y, z); // 5, 5
/*
            GO{
                a:1, 
                b:0,
                c:0,
                add:function add(n) {
            return n = n * 5;
        }
            }
        */

8.

++[[]][+[]] + [+[]];
/*
            ++[[]][0] + [0]
            ++[] + [0]
            '1' + '0'
            '10'
        */

9.

var name = 'This Window';
var obj = {
    name: 'My Object',
    getNameFun: function() { //AO{this:obj, name:'My Object', getNameFun: fn}
        return function() { //AO{this:window}
            return this.name; // 'This Window'
        }
    }
}
var result = obj.getNameFun();
console.log(result());
// GO{name: 'This Window', obj:{}}

10. 作用域链 = 函数执行时的AO对象 + 函数创建时的环境

var name = 'This Window';
var obj = {
    name: 'My Object',
    getNameFun: function() { //AO{this:obj, that: this}
        var that = this;
        return function() { //AO{this:window}
            return that.name; // this.name = 'My Object'
        }
    }
}
var result = obj.getNameFun();
console.log(result());
// GO{name: 'This Window', obj:{}}

11.

var length = 10;

function fn() {
    console.log(this.length);
}
var obj = {
    length: 5,
    method: function(fn) { //AO{this:obj}
        console.log(this.length); // 5
        fn(); //Ao{this:window} // 10
        arguments[0](); // fn() // AO{this: arguments}  //2
    }
}
obj.method(fn, 1);

12. 引用值操作地址

var a = {
    n: 1
}; // a: 地址1
var b = a; //b: 地址1
a.n = a = {
    m: 1
}; // a = 地址2 , 地址1.n = 地址2   // a = {m:1} , b.n = {m:1}  //b = {n:{m:1}}
console.log(a); //{m:1}
console.log(b); // {n:{m:1}}