Intermediate JavaScript thinking interview questions (a)

Answer Analysis at the bottom

1, Q: What is the following code will print Why??

window.a || (window.a ='1');
console.log(window.a);

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2, Q: What The following code will print Why??

var a = false + 1 ;
console.log(a);

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3, Q: What is the following code will print Why??

var b = false === 1;
console.log(b);

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4, Q: The following code will print what results Why??

if (typeof(a) && (-true) + (+undefined) + '') {
  console.log('结果为真');
}else{
  console.log('结果为假');
}

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5, Q: The following code will print what results Why??

var a = !!'' + !!' ' - !!false + '1' || '结果为假';
console.log(a);

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6, Q: The following code will print what results Why??

var fn = (
  function fn1() {
    return 1;
  },
  function fn2() {
    return 2;
  }
)();
console.log(fn);

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7, Q: The following code will print what results Why??

var a = 10;
if (function b() {}) {
  a += typeof b;
}
console.log(a);

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8, Q: What is the result of the following few prints Why??

function fn() {
  var marty = {
    name: 'marty',
    printName: function () {
      console.log(this.name);
    }
  }
  var test1 = {
    name: 'test1'
  }
  var test2 = {
    name: 'test2'
  }
  var test3 = {
    name: 'test3'
  }
  test3.printName = marty.printName;
  marty.printName.call(test1);//      1:
  marty.printName.apply(test2);//     2:
  marty.printName();//                3:
  test3.printName();//                4:
}

fn();

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9, Q: The following code will print what results Why??

var bar = {
  a: '1'
}

function fn() {
  bar.a = 'a';
  Object.prototype.b = 'b';
  return function () {
    console.log(bar.a);
    console.log(bar.b);
  }
}

fn()();

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• 1, the answer is:“1”

  Resolution:原理在于小括号的优先级高于其他运算符, 所以先赋值 window.a='1' 然后在进行或运算。

• 2, the answer:1

  Resolution:隐式类型转换 false 为 0 ,0 + 1 结果为1

• 3, the answer is:false

  Resolution:隐式类型转换false 为 0

• 4, the answer is:结果为真

  Resolution:typeof(a) 为 “undefined” ,隐式类型转换 (-true) 为 -1 ,(+undefined) 为 NaN 。-1 + NaN + '' = “NaN” , 所以结果为真

• 5. Answer:“11”

  Analysis: 隐式类型转换 !!'' 为 0 , !!' ' 字符串空格为 1 , !!false 为 0 , '1' , 1 + “1” =“11”
  Note:对字符串和数字进行加法运算，数字转成字符串！

• 6. Answer:2

  Resolution:逗号作为运算符使用, 结果为最后一个fn2() 。立即执行fn2()输出返回结果

• 7. Answer:“10undefined”

  Resolution:(function b() {}) 是作为表达式使用,直接忽略掉函数名 。所以b 是一个未定义的状态

• 8, the answer is:test1 test2 marty test3

  Resolution:这是一个this指向的问题：1、2 分别被 call()、apply() 改变了this的指向 3 、正常调用对象方法。4、将方法赋值给test3调用this指向test3

• 9, the answer is:a b

  Resolution:这里形成了一个闭包 返回一个函数 , fn() 调用时修改bar.a的值。bar.b 在原型链上寻找到顶级Object.prototype的时候获得 b值