Javascript小白经典题型(一)

1. 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

2. 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 和 0 1 2
• B: 0 1 2 和 3 3 3
• C: 3 3 3 和 0 1 2

3. 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

4. 输出是什么？

+true;
!"Lydia";

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

5. 哪一个是正确的？

const bird = {
  size: 'small'
}

const mouse = {
  name: 'Mickey',
  small: true
}

• A: mouse.bird.size是无效的
• B: mouse[bird.size]是无效的
• C: mouse[bird["size"]]是无效的
• D: 以上三个选项都是有效的

6. 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

7. 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)

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

8. 输出是什么？

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

  constructor({ newColor = 'green' } = {}) {
    this.newColor = newColor
  }
}

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

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

9. 输出是什么？

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

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

10. 当我们这么做时，会发生什么？

function bark() {
console.log('Woof!')
}

bark.animal = 'dog'

• A: 正常运行!
• B: SyntaxError. 你不能通过这种方式给函数增加属性。
• C: undefined
• D: ReferenceError

11. 输出是什么？

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

const member = new Person("Lydia", "Hallie");
Person.getFullName = function () {
  return `${this.firstName} ${this.lastName}`;
}

console.log(member.getFullName());

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

12. 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

13. 事件传播的三个阶段是什么？

• A: Target > Capturing > Bubbling
• B: Bubbling > Target > Capturing
• C: Target > Bubbling > Capturing
• D: Capturing > Target > Bubbling

14. 所有对象都有原型。

• A: true
• B: false

15. 输出是什么？

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

sum(1, '2')

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

16. 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

17. 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

18. 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

19. 输出是什么？

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

getAge(21)

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

20. 输出是什么？

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

getAge()

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

21. 输出是什么？

const sum = eval('10*10+5')

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

22. cool_secret 可访问多长时间？

sessionStorage.setItem('cool_secret', 123)

• A: 永远，数据不会丢失。
• B: 当用户关掉标签页时。
• C: 当用户关掉整个浏览器，而不只是关掉标签页。
• D: 当用户关闭电脑时。

23. 输出是什么？

var num = 8
var num = 10

console.log(num)

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

24. 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

25. 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

26. JavaScript 全局执行上下文为你做了两件事：全局对象和 this 关键字。

• A: true
• B: false
• C: it depends

27. 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

28. 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

29. 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

30. 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

31. 当点击按钮时，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: Outer div
• B: Inner div
• C: button
• D: 一个包含所有嵌套元素的数组

32. 当您单击该段落时，日志输出是什么？

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

• A: p div
• B: div p
• C: p
• D: div

33. 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

34. 输出是什么？

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

typeof sayHi()

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

35. 下面哪些值是 falsy?

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: All of them are falsy

36. 输出是什么？

console.log(typeof typeof 1)

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

37. 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

38. 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

39. JavaScript 中的一切都是？

• A: 基本类型与对象
• B: 函数与对象
• C: 只有对象
• D: 数字与对象

40. 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]

41. 输出是什么？

!!null
!!''
!!1

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

42. setInterval 方法的返回值是什么？

setInterval(() => console.log('Hi'), 1000)

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

43. 输出是什么？

[...'Lydia']

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

44. 输出是什么?

function* generator(i) {
  yield i;
  yield i * 2;
}

const gen = generator(10);

console.log(gen.next().value);
console.log(gen.next().value);

• A: [0, 10], [10, 20]
• B: 20, 20
• C: 10, 20
• D: 0, 10 and 10, 20

45. 返回值是什么?

const firstPromise = new Promise((res, rej) => {
  setTimeout(res, 500, "one");
});

const secondPromise = new Promise((res, rej) => {
  setTimeout(res, 100, "two");
});

Promise.race([firstPromise, secondPromise]).then(res => console.log(res));

• A: "one"
• B: "two"
• C: "two" "one"
• D: "one" "two"

46. 输出是什么?

let person = { name: "Lydia" };
const members = [person];
person = null;

console.log(members);

• A: null
• B: [null]
• C: [{}]
• D: [{ name: "Lydia" }]

47. 输出是什么?

const person = {
  name: "Lydia",
  age: 21
};

for (const item in person) {
  console.log(item);
}

• A: { name: "Lydia" }, { age: 21 }
• B: "name", "age"
• C: "Lydia", 21
• D: ["name", "Lydia"], ["age", 21]

48. 输出是什么?

console.log(3 + 4 + "5");

• A: "345"
• B: "75"
• C: 12
• D: "12"

49. num的值是什么?

const num = parseInt("7*6", 10);

• A: 42
• B: "42"
• C: 7
• D: NaN

50. 输出是什么?

[1, 2, 3].map(num => {
  if (typeof num === "number") return;
  return num * 2;
});

• A: []
• B: [null, null, null]
• C: [undefined, undefined, undefined]
• D: [ 3 x empty ]

原文链接：https://github.com/lydiahallie/javascript-questions/blob/master/zh-CN/README-zh_CN.md