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https://www.jschallenger.com/javascript-fundamentals/javascript-arrays

Fundamentals 編

JSchallenger にチャレンジ - Fundamentals / Basics
JSchallenger にチャレンジ - Fundamentals / Arrays
JSchallenger にチャレンジ - Fundamentals / Objects
JSchallenger にチャレンジ - Fundamentals / Dates
JSchallenger にチャレンジ - Fundamentals / Sets

DOM 編

JSchallenger にチャレンジ - DOM / DOM selector methods
JSchallenger にチャレンジ - DOM / Events and user interactions
JSchallenger にチャレンジ - DOM / DOM manipulation
JSchallenger にチャレンジ - DOM / DOM fundamentals
JSchallenger にチャレンジ - DOM / Recursive functions

Get nth element of array

Write a function that takes an array (a) and a value (n) as argument
Return the nth element of 'a'

https://www.jschallenger.com/javascript-fundamentals/javascript-arrays/get-nth-element-array-javascript

回答

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

Remove first n elements of an array

Write a function that takes an array (a) as argument
Remove the first 3 elements of 'a'
Return the result

回答

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

関連項目

• Array.prototype.slice() - JavaScript - MDN Web Docs

Get last n elements of an array

Write a function that takes an array (a) as argument
Extract the last 3 elements of a
Return the resulting array

回答

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

関連項目

• Array.prototype.slice() - JavaScript - MDN Web Docs

Get first n elements of an array

Write a function that takes an array (a) as argument
Extract the first 3 elements of a
Return the resulting array

回答

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

関連項目

• Array.prototype.slice() - JavaScript - MDN Web Docs

Return last n array elements

Write a function that takes an array (a) and a number (n) as arguments
It should return the last n elements of a

回答

function myFunction(a, n) {  
  return a.slice(-n)
}

関連項目

• Array.prototype.slice() - JavaScript - MDN Web Docs

Remove a specific array element

Write a function that takes an array (a) and a value (b) as argument
The function should clean a from all occurrences of b
Return the filtered array

回答

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

関連項目

• Array.prototype.filter() - JavaScript - MDN Web Docs

Count number of elements in JavaScript array

Write a function that takes an array (a) as argument
Return the number of elements in a

回答

function myFunction(a) {
  return a.length;
}

関連項目

• Array.length - JavaScript - MDN Web Docs

Count number of negative values in array

Write a function that takes an array of numbers as argument
Return the number of negative values in the array

回答

function myFunction(a) {
  return a.filter(cur => cur < 0).length
}

関連項目

• Array.prototype.filter() - JavaScript - MDN Web Docs
• Array.length - JavaScript - MDN Web Docs

Sort an array of numbers in descending order

Write a function that takes an array of numbers as argument
It should return an array with the numbers sorted in descending order

回答

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

関連項目

• Array.prototype.sort() - JavaScript - MDN Web Docs
• Array.prototype.reverse() - JavaScript - MDN Web Docs

Sort an array of strings alphabetically

Write a function that takes an array of strings as argument
Sort the array elements alphabetically
Return the result

回答

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

関連項目

• Array.prototype.sort() - JavaScript - MDN Web Docs

Return the average of an array of numbers

Write a function that takes an array of numbers as argument
It should return the average of the numbers

回答

function myFunction(arr) {
  return arr.reduce((acc, cur) => acc + cur) / arr.length;
}

関連項目

• Array.prototype.reduce() - JavaScript - MDN Web Docs

Return the longest string from an array of strings

Write a function that takes an array of strings as argument
Return the longest string

回答

function myFunction(arr) {
  return arr.reduce((prev, current) => 
    prev.length > current.length ? prev : current)
}

関連項目

• Array.prototype.reduce() - JavaScript - MDN Web Docs
• Array.length - JavaScript - MDN Web Docs
• 条件 (三項) 演算子 - JavaScript - MDN Web Docs

Array - Check if all array elements are equal

Write a function that takes an array as argument
It should return true if all elements in the array are equal
It should return false otherwise

回答

function myFunction(arr) {
  return Boolean(
    arr.reduce((acc, cur) => (cur === acc ? cur : false)),
  );
}

作者回答

function myFunction( arr ) {
  return new Set(arr).size === 1
}

Array - Merge an arbitrary number of arrays

Write a function that takes arguments an arbitrary number of arrays
It should return an array containing the values of all arrays

function myFunction(...arrays) {
  return arrays.flat()
}

Array - Sort array by object property

Write a function that takes an array of objects as argument
Sort the array by property b in ascending order
Return the sorted array

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

Array - Merge two arrays with duplicate values

Write a function that takes two arrays as arguments
Merge both arrays and remove duplicate values
Sort the merge result in ascending order
Return the resulting array

回答

function myFunction(a, b) {
  return Array.from(
    new Set(a.concat(b).sort((a, b) => a - b)),
  );
}

Array - Sum up all array elements with values greater than

Write a function that takes an array (a) and a number (b) as arguments
Sum up all array elements with a value greater than b
Return the sum

回答

function myFunction(a, b) {
  return a.reduce(
    (prev, cur) => (cur > b ? prev + cur : prev),
    0,
  );
}

Array - Create a range of numbers

Write a function that takes two numbers (min and max) as arguments
Return an array of numbers in the range min to max

回答

function myFunction(min, max) {
  const array = [...new Array(max - min + 1)];
  return array.map((_, index) => index + min)
}

Array - Group array of strings by first letter

Write a function that takes an array of strings as argument
Group those strings by their first letter
Return an object that contains properties with keys representing first letters
The values should be arrays of strings containing only the corresponding strings
For example, the array ['Alf', 'Alice', 'Ben'] should be transformed to
{ a: ['Alf', 'Alice'], b: ['Ben']}

回答

function myFunction(arr) {
  const result =  arr.reduce((prev, current) => {
    const initial = current.split('')[0].toLowerCase();
    return { 
      ...prev, 
      [initial]: [...(prev[initial] ?? []), current] 
    };
  }, {});
  return result;
}

Array - Define an array with conditional elements

Write a function that takes an array with arbitrary elements and a number as arguments
Return a new array, the first element should be either the given number itself
or zero if the number is smaller than 6
The other elements should be the elements of the original array
Try not to mutate the original array

回答

function myFunction(arr, num) {
  return num >= 6 ? [num].concat(arr) : [0].concat(arr)
}

Array - Get every nth element of array

Write a function that takes an array (a) and a value (n) as arguments
Save every nth element in a new array
Return the new array

回答

function myFunction(a, n) 
  return a.filter((_, i) => (i+1) % n === 0)
}

Calculate the sum of an array of numbers

Write a function that takes an array of numbers as argument
It should return the sum of the numbers

回答

function myFunction(a) 
  a.reduce((acc, cur) => acc + cur)
}