前端笔试题

# 前端笔试题

var str1 = "hello world";

var str2 = String("hello world");

var str3 = new String("hello world");

console.log(str1 === str2); // true

console.log(str1 === str3); // false

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# parseInt

['10', '10', '10', '10', '10'].map(parseInt) 的输出值是什么？[10, NaN, 2, 3, 4]

['1','2','3'].map(parseInt) [1, NaN, NaN]

# 运算

let a = (b = 10);
(function() {
  let a = (b = 20);
})();
console.log(a); // 10
console.log(b); // 20
// 连等操作是从右向左执行的

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# 闭包

var name = 1;
(function() {
  console.log(name + this.name); // browser: -> 'undefined1'; Node.js: -> NaN
  var name = "2";
  console.log(name + this.name); // browser: -> '21'; Node.js: -> '2undefined'
})();

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var b = 10;
(function b() {
  b = 20;
  console.log(b); // 输出：[Function: b]
})();
console.log(b); // 输出：10

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for (var i = 0; i < 5; i++) {
  setTimeout(() => {
    console.log(i);
  }, i);
} // 输出：5,5,5,5,5

for (let i = 0; i < 5; i++) {
  setTimeout(() => {
    console.log(i);
  }, i);
} // 输出：0,1,2,3,4

for (var i = 0; i < 5; i++) {
  setTimeout(
    (i) => {
      console.log(i);
    },
    i,
    i
  );
} // 输出 0,1,2,3,4

for (var i = 0; i < 5; i++) {
  (function(i) {
    setTimeout(
      (i) => {
        console.log(i);
      },
      i,
      i
    );
  })(i);
} // 自执行函数，输出 0,1,2,3,4

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# this

const person1 = {
  name: "person1",
  say: function() {
    console.log(this.name);
  },
  eat: () => {
    console.log(this.name);
  },
};

const person2 = {
  name: "person2",
};

person1.say(); // person1
person1.say.call(person2); // person2
person1.eat.call(person2); // undefined

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# typeof

// 这两种方式的区别？typeof 判断
const str1 = "abc";
const str2 = new String("abc");

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更新时间: 2/10/2022, 7:21:32 PM

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