43道JS面试题

1. 下面代码输出是什么?

function sayHi() {
  console.log(name);
  console.log(age);
  var name = "Lydia";
  let age = 21;
}

sayHi();

A: Lydia 和 undefined
B: Lydia 和 ReferenceError
C: ReferenceError 和 21
D: undefined 和 ReferenceError答案 ```````````` ``````````

关于 let、var和function:

let的’创建’过程被提升了，但是初始化没有提升。
var的’创建’和’初始化’都被提升了
function的’创建’、’初始化’和’赋值’都被提升了。

2. 下面代码的输出是什么?

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

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1);
}

A: 0 1 2 and 0 1 2
B: 0 1 2 and 3 3 3
C: 3 3 3 and 0 1 2

答案

3. 下面代码的输出是什么?

const shape = {
  radius: 10,
  diameter() {
    return this.radius * 2;
  },
  perimeter: () => 2 * Math.PI * this.radius
};

shape.diameter();
shape.perimeter();

A: 20 and 62.83185307179586
B: 20 and NaN
C: 20 and 63
D: NaN and 63

答案

4. 下面代码的输出是什么?

+true;
!"Lydia";

A: 1 and false
B: false and NaN
C: false and false

答案

5. 哪个选项是不正确的?

const bird = {
  size: "small"
};

const mouse = {
  name: "Mickey",
  small: true
};

A: mouse.bird.size
B: mouse[bird.size]
C: mouse[bird["size"]]
D: All of them are valid

答案

6. 下面代码的输出是什么?

let c = { greeting: "Hey!" };
let d;

d = c;
c.greeting = "Hello";
console.log(d.greeting);

A: Hello
B: undefined
C: ReferenceError
D: TypeError

答案

7. 下面代码的输出是什么?

let a = 3;
let b = new Number(3);
let c = 3;

console.log(a == b);
console.log(a === b);
console.log(b === c);

答案

8. 下面代码的输出是什么?

class Chameleon {
  static colorChange(newColor) {
    this.newColor = newColor;
  }

  constructor({ newColor = "green" } = {}) {
    this.newColor = newColor;
  }
}

const freddie = new Chameleon({ newColor: "purple" });
freddie.colorChange("orange");

A: orange
B: purple
C: green
D: TypeError

答案

9. 下面代码的输出是什么?

let greeting;
greetign = {}; // Typo!
console.log(greetign);

A: {}
B: ReferenceError: greetign is not defined
C: undefined

答案

10. 当我们这样做时会发生什么?

function bark() {
  console.log("Woof!");
}

bark.animal = "dog";

A: Nothing, this is totally fine!
B: SyntaxError. You cannot add properties to a function this way.
C: undefined
D: ReferenceError

答案

11. 下面代码的输出是什么?

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

const member = new Person("Lydia", "Hallie");
Person.getFullName = () => this.firstName + this.lastName;

console.log(member.getFullName());

A: TypeError
B: SyntaxError
C: Lydia Hallie
D: undefined undefined

答案

12. 下面代码的输出是什么?

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

const lydia = new Person("Lydia", "Hallie");
const sarah = Person("Sarah", "Smith");

console.log(lydia);
console.log(sarah);

A: Person {firstName: "Lydia", lastName: "Hallie"} and undefined
B: Person {firstName: "Lydia", lastName: "Hallie"} and Person {firstName: "Sarah", lastName: "Smith"}
C: Person {firstName: "Lydia", lastName: "Hallie"} and {}
D: Person {firstName: "Lydia", lastName: "Hallie"} and ReferenceError

答案

13. 所有对象都有原型.

A: 对
B: 错误

答案

14. 下面代码的输出是什么?

function sum(a, b) {
  return a + b;
}

sum(1, "2");

A: NaN
B: TypeError
C: "12"
D: 3

答案

15. 下面代码的输出是什么?

let number = 0;
console.log(number++);
console.log(++number);
console.log(number);

A: 1 1 2
B: 1 2 2
C: 0 2 2
D: 0 1 2

答案

16. 下面代码的输出是什么?

function getPersonInfo(one, two, three) {
  console.log(one);
  console.log(two);
  console.log(three);
}

const person = "Lydia";
const age = 21;

getPersonInfo`${person} is ${age} years old`;

A: Lydia 21 ["", "is", "years old"]
B: ["", "is", "years old"] Lydia 21
C: Lydia ["", "is", "years old"] 21

答案

17. 下面代码的输出是什么?

function checkAge(data) {
  if (data === { age: 18 }) {
    console.log("You are an adult!");
  } else if (data == { age: 18 }) {
    console.log("You are still an adult.");
  } else {
    console.log(`Hmm.. You don't have an age I guess`);
  }
}

checkAge({ age: 18 });

A: You are an adult!
B: You are still an adult.
C: Hmm.. You don't have an age I guess

答案

18. 下面代码的输出是什么?

function getAge(...args) {
  console.log(typeof args);
}

getAge(21);

A: "number"
B: "array"
C: "object"
D: "NaN"

答案

19. 事件传播的三个阶段是什么？?

A: 目标 > 捕获 > 冒泡
B: 冒泡 > 目标 > 捕获
C: 目标 > 冒泡 > 捕获
D: 捕获 > 目标 > 冒泡

答案

20. 下面代码的输出是什么?

function getAge() {
  "use strict";
  age = 21;
  console.log(age);
}

getAge();

A: 21
B: undefined
C: ReferenceError
D: TypeError

答案

21. 下面代码的输出是什么?

const sum = eval("10*10+5");

A: 105
B: "105"
C: TypeError
D: "10\*10+5"

答案

22. cool_secret 可以访问多长时间?

sessionStorage.setItem("cool_secret", 123);

A：永远，数据不会丢失。 B：用户关闭选项卡时。 C：当用户关闭整个浏览器时，不仅是选项卡。 D：用户关闭计算机时。

答案

23. 下面代码的输出是什么?

var num = 8;
var num = 10;

console.log(num);

A: 8
B: 10
C: SyntaxError
D: ReferenceError

答案

24. 下面代码的输出是什么?

const obj = { 1: "a", 2: "b", 3: "c" };
const set = new Set([1, 2, 3, 4, 5]);

obj.hasOwnProperty("1");
obj.hasOwnProperty(1);
set.has("1");
set.has(1);

A: false true false true
B: false true true true
C: true true false true
D: true true true true

答案

25. 下面代码的输出是什么?

const obj = { a: "one", b: "two", a: "three" };
console.log(obj);

A:{ a: "one", b: "two" }
B: { b: "two", a: "three" }
C: { a: "three", b: "two" }
D: SyntaxError

答案

26. JavaScript 全局执行上下文为你创建了两个东西:全局对象和 this 关键字.

A: 对
B: 错误
C: 视情况而定

答案

27. 下面代码的输出是什么?

for (let i = 1; i < 5; i++) {
  if (i === 3) continue;
  console.log(i);
}

A: 1 2
B: 1 2 3
C: 1 2 4
D: 1 3 4

答案

28. 下面代码的输出是什么?

String.prototype.giveLydiaPizza = () => {
  return "Just give Lydia pizza already!";
};

const name = "Lydia";

name.giveLydiaPizza();

A: "Just give Lydia pizza already!"
B: TypeError: not a function
C: SyntaxError
D: undefined

答案

29. 下面代码的输出是什么?

const a = {};
const b = { key: "b" };
const c = { key: "c" };

a[b] = 123;
a[c] = 456;

console.log(a[b]);

A: 123
B: 456
C: undefined
D: ReferenceError

答案

30. 下面代码的输出是什么?

const foo = () => console.log("First");
const bar = () => setTimeout(() => console.log("Second"));
const baz = () => console.log("Third");

bar();
foo();
baz();

A: First Second Third
B: First Third Second
C: Second First Third
D: Second Third First

答案

31. 单击按钮时 event.target 是什么?

<div onclick="console.log('first div')">
  <div onclick="console.log('second div')">
    <button onclick="console.log('button')">
      Click!
    </button>
  </div>
</div>

A: div 外部
B: div 内部
C: button
D: 所有嵌套元素的数组.

答案

32. 单击下面的 html 片段打印的内容是什么?

<div onclick="console.log('div')">
  <p onclick="console.log('p')">
    Click here!
  </p>
</div>

A: p div
B: div p
C: p
D: div

答案

33. 下面代码的输出是什么?

const person = { name: "Lydia" };

function sayHi(age) {
  console.log(`${this.name} is ${age}`);
}

sayHi.call(person, 21);
sayHi.bind(person, 21);

A: undefined is 21 Lydia is 21
B: function function
C: Lydia is 21 Lydia is 21
D: Lydia is 21 function

答案

34. 下面代码的输出是什么?

function sayHi() {
  return (() => 0)();
}

typeof sayHi();

A: "object"
B: "number"
C: "function"
D: "undefined"

答案

35. 下面这些值哪些是假值?

0;
new Number(0);
("");
(" ");
new Boolean(false);
undefined;

A: 0, '', undefined
B: 0, new Number(0), '', new Boolean(false), undefined
C: 0, '', new Boolean(false), undefined
D: 所有都是假值

答案

36. 下面代码的输出是什么?

console.log(typeof typeof 1);

A: "number"
B: "string"
C: "object"
D: "undefined"

答案

37. 下面代码的输出是什么?

const numbers = [1, 2, 3];
numbers[10] = 11;
console.log(numbers);

A: [1, 2, 3, 7 x null, 11]
B: [1, 2, 3, 11]
C: [1, 2, 3, 7 x empty, 11]
D: SyntaxError

答案

38. 下面代码的输出是什么?

(() => {
  let x, y;
  try {
    throw new Error();
  } catch (x) {
    (x = 1), (y = 2);
    console.log(x);
  }
  console.log(x);
  console.log(y);
})();

A: 1 undefined 2
B: undefined undefined undefined
C: 1 1 2
D: 1 undefined undefined

答案

39. JavaScript 中的所有内容都是…

A：原始或对象
B：函数或对象
C：技巧问题！只有对象
D：数字或对象

答案

40. 下面代码的输出是什么?

[[0, 1], [2, 3]].reduce(
  (acc, cur) => {
    return acc.concat(cur);
  },
  [1, 2]
);

A: [0, 1, 2, 3, 1, 2]
B: [6, 1, 2]
C: [1, 2, 0, 1, 2, 3]
D: [1, 2, 6]

答案

41. 下面代码的输出是什么?

!!null;
!!"";
!!1;

A: false true false
B: false false true
C: false true true
D: true true false

答案

42. setInterval 方法的返回值什么?

setInterval(() => console.log("Hi"), 1000);

A：一个唯一的 id
B：指定的毫秒数
C：传递的函数
D：undefined

答案

43. 下面代码的返回值是什么?

[..."Lydia"];

A: ["L", "y", "d", "i", "a"]
B: ["Lydia"]
C: [[], "Lydia"]
D: [["L", "y", "d", "i", "a"]]

答案

答案: A 字符串是可迭代的。扩展运算符将迭代的每个字符映射到一个元素

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