The answer

The answer

Js object knowledge examination

1. Which one is true?

const bird = {
  size: 'small'
}

const mouse = {
  name: 'Mickey'.small: true
}
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  • A: mouse.bird.sizeIs invalid
  • B: mouse[bird.size]Is invalid
  • C: mouse[bird["size"]]Is invalid
  • D: All three of the above options are valid

2. What is the output


const a = {}
const b = { key: 'b' }
const c = { key: 'c' }

a[b] = 123
a[c] = 456
console.log(a[b])
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  • A: 123
  • B: 456
  • C: undefined
  • D: ReferenceError

3. What is printed next

var obj = {}; 
console.log(delete obj.p)
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  • A: true
  • B: false
  • C: error
  • D: undefined

4. What does the following value print out

let obj = {name:'jimmy'.age:18}
let obj1 = obj;
obj.name = "chimmy"
console.log(obj1)
obj1 = {}
obj.age = 22

console.log(obj1)

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  • {name:’ Jimmy ‘,age:18}, {name:’ Jimmy ‘,age:18}
  • B: {name: ‘chimmy, age: 18}, {}
  • C: {name: ‘chimmy, age: 18}, {name:’ chimmy, age: 18}
  • D: {name: ‘chimmy, age: 18}, {name:’ chimmy, age: 22}

4.1 What is the output

const obj = { a: 'one'.b: 'two'.a: 'three' }
console.log(obj)
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  • A: { a: "one", b: "two" }
  • B: { b: "two", a: "three" }
  • C: { a: "three", b: "two" }
  • D: SyntaxError

The above five questions are to investigate the basic knowledge of the object, if not all right, you can go to see my article JS object basic knowledge, review the basic knowledge of JS objects.

Data types and conversions

5. Please write the answers below

1.Js so far how many data types, please write out.2. NaN= = =NaN 
3. typeof NaN
4. typeof null
5. typeof typeof 1
6. null= =undefined
7. ""= =0
8. 1 + null
9. '1' + null
10. Number(null)
11. Number("")
12.[] = =0
13.[] = =! []14.[] = =false
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If you are not very good at it, you can see how much I know about js data types

Knowledge about wrapper objects

6. What is the output?

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

console.log(a == b)
console.log(a === b)
console.log(b === c)
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  • A: true false true
  • B: false false true
  • C: true false false
  • D: false true true

7. Those values are false

0
new Number(0)
(' ')
new Boolean(false)
undefined
new Boolean(false).valueOf()
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  • A: 0.' '.undefined.new Boolean(false).valueOf()
  • B: 0.new Number(0).' '.new Boolean(false).undefined
  • C: 0.' '.new Boolean(false).undefined
  • D: all is false

If you are not very good at this, you can check out my article

Array Length

8. The following statement is true

let arr = [1.2.3.4.5];
let arr2 = [1.3]
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  • Arr. length = 3, and the array is [1,2,3]
  • B: Execute arr[10] = 11; In this case, arr.length is 6
  • C: Run delete arr[2]; Arr. length = 4, array = [1,2,4,5]
  • D: arr2.length is 2

The above question is the array length related knowledge, if you master is not very good, you can look at my js array

Function points

9. What is printed correctly below is

var f = function () { console.log('1'); } 
function f() { console.log('2'); } 
f()
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  • A:1
  • B:2
  • C: error
  • D: undefined

10. What is printed correctly below is

var a = 1; 
var x = function () { 
    console.log(a);
};
function f() {
   var a = 2; 
   x();
} 
f() 
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  • 2 A:
  • B: 1.
  • C: error
  • D: undefined

The above question is the function name promotion and function scope related knowledge, if you are not very good, you can see my js function

Event Cycle knowledge points

11. The following output is correct

console.log('Start execution');
setTimeout(() = > {
  console.log('timeout1')},0);
console.log("It's over.")
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  • A: Start execution, timeout1, end
  • B: Timeout1, start execution, end
  • C: Start execution, end, timeout1

12. The following output is correct

Promise.resolve().then(() = > {
  console.log('Promise1')
  setTimeout(function () {
    console.log("setTimeout1")},10)})setTimeout(() = > {
  console.log('setTimeout2')
  Promise.resolve().then(() = > {
    console.log('Promise2')})},10)
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  • A: Promise1, setTimeout1, setTimeout2, Promise2
  • B: Promise1, Promise2, setTimeout1, setTimeout2
  • C: Promise1, setTimeout2, setTimeout1, Promise2
  • D: setTimeout2, Promise2, setTimeout1

Js Event Loop(Event Loop) js Event Loop(Event Loop) js Event Loop(Event Loop

Prototypes and prototype chains

13. The value printed below is

function A(){};
A.prototype.n = 1;
var b = new A();
A.prototype = {
  n: 2.m: 3
}
var c = new A();

console.log(b.n);
console.log(b.m);
console.log(c.n);
console.log(c.m);

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If you don’t understand this point, please refer to my article, which is explained at the end of the article.

This points to the question

14. What is the name value printed below

var name = "jimmy";
var a = {
    name: "chimmy".fn : function () {
        console.log(this.name); 
    }
}
a.fn();
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15 The values printed below are, in order

var name = 'jimmy';
var student = {
    name: 'chimmy'.doSth: function(){
      console.log(this.name); 
    },
    doSth2: function(){
       var arrowDoSth = () = > {
           console.log(this.name);
       }
       arrowDoSth();
    },
    doSth3: () = > {
       console.log(this.name); 
    }
}
student.doSth();
student.doSth2();
student.doSth3();

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Please refer to the passage “How much do you know about this?” for these two questions, they are taken from the passage.

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