Javascript Basics のまとめ – Javascript Fundamentals – JSchallenger

はじめに

JSchallenger の Javascript Fundamentals – Javascript Basics のまとめです。
まとめページのトップは、JSchallenger まとめです。

問題一覧と回答

Sum two numbers

// Write a function that takes two numbers (a and b) as argument
// Sum a and b
// Return the result

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

Comparison operators, strict equality

// Write a function that takes two values, say a and b, as arguments
// Return true if the two values are equal and of the same type

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

Get type of value

// Write a function that takes a value as argument
// Return the type of the value

function myFunction(a) {
   return typeof a;
}

Get nth character of string

// Write a function that takes a string (a) and a number (n) as argument
// Return the nth character of ‘a’

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

Remove first n characters of string

// Write a function that takes a string (a) as argument
// Remove the first 3 characters of a
// Return the result

substring を使ってしまったが、slice を使うようにした方が応用が利いてよさそう

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

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

Get last n characters of string

// Write a function that takes a string as argument
// Extract the last 3 characters from the string
// Return the result

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

Get first n characters of string

// Write a function that takes a string (a) as argument
// Get the first 3 characters of a
// Return the result

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

Extract first half of string

// Write a function that takes a string (a) as argument
// Extract the first half a
// Return the result

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

Remove last n characters of string

// Write a function that takes a string (a) as argument
// Remove the last 3 characters of a
// Return the result

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

Return the percentage of a number

// Write a function that takes two numbers (a and b) as argument
// Return b percent of a

a の b% は？という問題なので、a に b をかけるというのが普通の考え方だと思うのだが、これって日本的な考え方なのか？
まあ、細かい話だが。

function myFunction(a, b) {
   return a * b / 100;
}

function myFunction(a, b) {
  return b / 100 * a
}

Basic JavaScript math operators

// Write a function that takes 6 values (a,b,c,d,e,f) as arguments
// Sum a and b
// Then substract by c
// Then multiply by d and divide by e
// Finally raise to the power of f and return the result
// Tipp: mind the order

括弧がないだけで同じだけど、それに括弧つけるなら substract も括弧を付けるべきでは？

function myFunction(a, b, c, d, e, f) {
   return ((a + b - c) * d / e) ** f;
}

function myFunction(a, b, c, d, e, f) {
   return (((a + b - c) * d) / e) ** f;
}

Check whether a string contains another string and concatenate

// Write a function that takes two strings (a and b) as arguments
// If a contains b, append b to the beginning of a
// If not, append it to the end
// Return the concatenation

function myFunction(a, b) {
  return a.indexOf(b) === -1 ? a + b : b + a
}

Check if a number is even

// Write a function that takes a number as argument
// If the number is even, return true
// Otherwise, return false

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

How many times does a character occur?

// Write a function that takes two strings (a and b) as arguments
// Return the number of times a occurs in b

これは圧倒的に author’s solution の方がきれいな回答だった(/_＼*)

function myFunction(a, b) {
   return (b.match(newRegExp(a.replace('?', '\\?'), 'g')) || [] ).length;
}

function myFunction(a, b) {
  return b.split(a).length - 1
}

Check if a number is a whole number

// Write a function that takes a number (a) as argument
// If a is a whole number (has no decimal place), return true
// Otherwise, return false

function myFunction(a) {
   returnNumber.isInteger(a);
}

function myFunction(a) {
  return a - Math.floor(a) === 0
}

Multiplication, division, and comparison operators

// Write a function that takes two numbers (a and b) as arguments
// If a is smaller than b, divide a by b
// Otherwise, multiply both numbers
// Return the resulting value

function myFunction(a, b) {
  return a < b ? a / b : a * b
}

Round a number to 2 decimal places

// Write a function that takes a number (a) as argument
// Round a to the 2nd digit after the comma
// Return the rounded number

toFixed 知らんかった。

function myFunction(a) {
   returnMath.round(a * 100) / 100;
}

function myFunction(a) {
  returnNumber(a.toFixed(2));
}

Split a number into its digits

// Write a function that takes a number (a) as argument
// Split a into its individual digits and return them in an array
// Tipp: you might want to change the type of the number for the splitting

このケースは連結して書いた方が分かりやすい気がする。

function myFunction(a) {
   returnString(a).split('').map(s => Number(s));
}

function myFunction( a ) {
  const string = a + '';
  const strings = string.split('');
  return strings.map(digit => Number(digit))
}

Clear up the chaos behind these strings

// It seems like something happened to these strings
// Can you figure out how to clear up the chaos?
// Write a function that joins these strings together such that they form the following words: // ‘Javascript’, ‘Countryside’, and ‘Downtown’
// You might want to apply basic JS string methods such as replace(), split(), slice() etc

function myFunction(a, b) {
   return a[0].toUpperCase() +
	   a.slice(1).replace('%', '') +
	   b.replace('%', '').split('').reverse().join('');
}

function myFunction(a, b) {
  constfunc = x => x.replace('%','');
  const first = func(a);
  const second = func(b).split('').reverse().join('');
  return first.charAt(0).toUpperCase() + first.slice(1) + second;
}

Return the next higher prime number

// This challenge is a little bit more complex
// Write a function that takes a number (a) as argument
// If a is prime, return a
// If not, return the next higher prime number

function の中に function を作る発想はなかった。

function myFunction(a) {
   while (1) {
      flag = true;
      for (i = 2; i <= Math.sqrt(a); i ++) {
         if (a % i === 0) {
            flag = false;
            break;
         }
      }
      if (flag) return a;
      a ++;
   }
}

function myFunction( a ) {
  function isPrime(num) {
    for (let i = 2; i <= Math.sqrt(num); i++) {
      if (num % i === 0) returnfalse;
    }
    return num > 1;
  }
  let n = a;
  while (!isPrime(n)) n++;
  return n
}

Find next higher natural number that is divisble by y

// Write a function that takes two numbers, say x and y, as arguments
// Check if x is divisible by y
// If yes, return x
// If not, return the next higher natural number that is divisible by y

計算で求めるのだと思いきや、泥臭い方法だった。

function myFunction(x, y) {
   return x % y === 0 ? x : (Math.floor(x / y) + 1) * y;
}

function myFunction(x, y) {
  while (x % y !== 0) x++;
  return x;
}

Insert character after every n characters (backwards)

// Write a function that takes two strings (a and b) as arguments
// Beginning at the end of ‘a’, insert ‘b’ after every 3rd character of ‘a’
// Return the resulting string

簡単なテストケースしか無いので正規表現で簡単にいける

function myFunction(a, b) {
   return a.replace( /(?=(...)+(?!.))/g, b);
}

function myFunction(a, b) {
   let result = [];
   let rest = a;
   while (rest.length) {
   	result.push(rest.slice(-3));
   	rest = rest.slice(0, -3);
   }
   return result.reverse().join(b);
}

Find the correct word by incrementing letters in alphabet

// Write a function that takes a string as argument
// As it is, the string has no meaning
// Increment each letter to the next letter in the alphabet
// Return the correct word

これも連結して書いた方が分かりやすい気がする。

function myFunction(str) {
   return str.split('').map(s => String.fromCharCode(s.charCodeAt() + 1)).join('');
}

function myFunction(str) {
  const arr = [...str];
  const correctedArray = arr.map(e => String.fromCharCode(e.charCodeAt()+1));
  return correctedArray.join('');
}

Javascript Basics のまとめ - Javascript Fundamentals - JSchallenger