一些有意思的JS题目

[YutHelloWorld]

在此摘录汇总一些从社区看到的文章、面经中的比较有意思的JS题目~

实例原型

Function.prototype.a = 'a';
Object.prototype.b = 'b';
function Person(){};
var p = new Person();
console.log('p.a: '+ p.a); // p.a: undefined
console.log('p.b: '+ p.b); // p.b: b

解释：实例p的原型链由Person、Object构成，没有Function

this

const person = {
  name: 'menglinghua',
  say: function (){
    return function (){
      console.log(this.name);
    };
  }
};
person.say()(); // ''(空字符串)

解释：this指向window，window.name===''

const person = {
  name: 'menglinghua',
  say: function (){
    return () => {
      console.log(this.name);
    };
  }
};
person.say()(); 

解释：箭头函数将this绑定到了声明域，指向person

队列

setTimeout(() => console.log('a'), 0);
var p = new Promise((resolve) => {
  console.log('b');
  resolve();
});
p.then(() => console.log('c'));
p.then(() => console.log('d'));
console.log('e');
// 结果：b e c d a

解释：任务队列优先级：promise.Trick()>promise的回调>setTimeout>setImmediate

console.log('1');

setTimeout(function() {
    console.log('2');
    process.nextTick(function() {
        console.log('3');
    })
    new Promise(function(resolve) {
        console.log('4');
        resolve();
    }).then(function() {
        console.log('5')
    })
})
process.nextTick(function() {
    console.log('6');
})
new Promise(function(resolve) {
    console.log('7');
    resolve();
}).then(function() {
    console.log('8')
})

setTimeout(function() {
    console.log('9');
    process.nextTick(function() {
        console.log('10');
    })
    new Promise(function(resolve) {
        console.log('11');
        resolve();
    }).then(function() {
        console.log('12')
    })
})

结果： 1 7 6 8 2 4 3 5 9 11 10 12
解释：原文地址，https://juejin.im/post/59e85eebf265da430d571f89#heading-4

数值类型转换

Number('') // 0
parseInt('') // NaN
Number('123abc') // NaN
parseInt('123abc') // 123

[vivinbear]

const person = {
name: 'menglinghua',
say: function (){
return function (){
console.log(this.name);
};
}
};
person.say()(); // 这里的输出结果需要根据环境来区分,浏览器下,是空字符串,而node中时undefined.因为浏览器环境下,window是有name属性的,name的值是空,所以会输出空字符串
其余的很棒