答案

1、字符串

1.sum two numbers

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

2.Comparison operators, strict equality

function myFunction(a, b)
{
return a === b
}

3.Get type of value

function myFunction(a)
{
return typeof(a)
}

4.Get nth character of string

function myFunction(a, n) {
  return a[n-1]
}

5.Remove first n characters of string

function myFunction(a) {
  return a.slice(3)
}

6.Get last n characters of string

function myFunction(str) {
  return str.slice(str.length-3)
}

7.Get first n characters of string

function myFunction(a) {
  return a.slice(0, 3)
}

8.Extract first half of string

function myFunction(a) {
  return a.slice(0, a.length / 2)
}

9.Remove last n characters of string

function myFunction(a) {
  return a.slice(0,a.length-3)
}

10.Check if a number is even

function myFunction(a) {
  return a % 2 === 0
}

2、数组

1.Get nth element of array

function myFunction(a, n) {
  return a[n-1]
}

2.Remove first n elements of an array

function myFunction(a) {
  return a.slice(3)
}

3.Get last n elements of an array

function myFunction(a) {
  return a.slice(-3)
}

4.Get first n elements of an array

function myFunction(a) {
  return a.slice(0,3)
}

5.Remove a specific array element

function myFunction(a,b) {
  return a.filter(item => item !== b)
}

6.Count number of elements in JavaScript array

function myFunction(a) {
  return a.length
}

7.Sort an array of strings alphabetically

function myFunction(arr) {
  return arr.sort()
}

8.Sort an array of numbers in descending order

function myFunction(arr) {
  return arr.sort((a, b) => b - a)
}

9.Calculate the sum of an array of numbers

function myFunction(a) {
  let sum = 0
  for(let i = 0;i<a.length;i++){
    sum += a[i]
  }
  return sum
}

3、对象

1.Accessing object properties one

function myFunction(obj) {
  return obj["country"];
}

2.Accessing object properties three

function myFunction(obj, key) {
    return obj[key];
}

3.Check if property exists in object

function myFunction(a, b) {
    return b in a;
}

4.Check if property exists in object and is truthy
5.Creating Javascript objects one

function myFunction(a) {
  return { key: a };
}

6.Creating Javascript objects two

function myFunction(a, b) {
 return {[a]:b}
}

7.Creating Javascript objects three

function myFunction(a, b) {
   let obj = {}
   for(let i = 0; i < a.length; i++){
      obj[a[i]] = b[i]
   }
   return obj
}

8.Extract keys from Javascript object

function myFunction(a) {
   return Object.keys(a)
}

9.Sum object values

function myFunction(a) {
   return eval(Object.values(a).join("+"))
}

10.Remove a property from an object

function myFunction(obj) {
   let {b,...item} = obj
   return item
}