<!-- {%hackmd BJrTq20hE %} -->
<!-- {%hackmd hackmd-dark-theme %} -->

# 8 kyu

### [Grasshopper - Summation](https://www.codewars.com/kata/55d24f55d7dd296eb9000030)
:::spoiler 解法

let summation = function (num) {
  let sum = 0;
  for(let i = 1; i<=num; i++){
    sum+=i;
  }
  return sum;
}

:::
### [Reversed Strings](https://www.codewars.com/kata/5168bb5dfe9a00b126000018)
:::spoiler 解法

function solution(str){
  return str.split("").reverse().join("");
}

let str = "abcdefg";

str = str.split("");
console.log(str); // ['a', 'b', 'c', 'd', 'e', 'f', 'g']

str = str.reverse();
console.log(str); // ['g', 'f', 'e', 'd', 'c', 'b', 'a']

str = str.join("");
console.log(str); // gfedcba

// str = str.join(" + ");
// console.log(str); // g + f + e + d + c + b + a

:::

### [Count by X](https://www.codewars.com/kata/5513795bd3fafb56c200049e)
:::spoiler 解法

function countBy(x, n) {
  let z = [];
  for(let i = 1; i<=n ; i++){
    let pushNum = x * i;
    z.push(pushNum);
  }
  return z;
}

:::

### [Beginner - Reduce but Grow](https://www.codewars.com/kata/57f780909f7e8e3183000078)
:::spoiler 解法

function grow(x){
  let result = 1;
  x.forEach(item=>{
    result *= item;
  })
  return result;
}

// const grow=x=> x.reduce((a,b) => a*b);

:::

### [Convert boolean values to strings 'Yes' or 'No'.](https://www.codewars.com/kata/53369039d7ab3ac506000467)
:::spoiler 解法

function boolToWord( bool ){
  return bool ? "Yes" : "No";
}

:::

### [Will there be enough space?](https://www.codewars.com/kata/5875b200d520904a04000003)

:::spoiler 解法

function enough(cap, on, wait) {
  let empty = cap - on - wait;
  return empty >= 0 ? 0 : Math.abs(empty) 
}

// function enough(cap, on, wait) {
//   return Math.max(wait + on - cap, 0);
// }

:::

### [Abbreviate a Two Word Name](https://www.codewars.com/kata/57eadb7ecd143f4c9c0000a3)*
- 知識點：[Array.prototype.map()](https://www.codewars.com/kata/57eadb7ecd143f4c9c0000a3)
:::spoiler 解法
- 該題傳入的值也會有單字開頭是小寫的情況

function abbrevName(name){
 return name.split(" ").map((c)=> c[0].toUpperCase()).join(".");
}

:::

### [Sum without highest and lowest number](https://www.codewars.com/kata/576b93db1129fcf2200001e6)***
- 知識點：!!轉布林、null型別、[Array.prototype.reduce()](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce)、Math.min
:::spoiler 解法
- 注意測試值包含 null ，null 為 object，所以只能是兩個等號

function sumArray(array) {
  if (array == null || array.length <= 2) {
    return 0;
  }
  array.sort((a, b) => {
    return a - b;
  });
  array = array.slice(1, array.length - 1);
  return array.reduce((a, c) => a + c);
// function sumArray(array) {
//   return Array.isArray(array) && array.length > 1
//     ? array.reduce((s, n) => s + n, 0) - Math.min(...array) - Math.max(...array)
//     : 0
// }
}

- !!轉為布林，單個!結果會相反

function sumArray(array) {
  return !!array;
}
console.log(sumArray(null)); // false
console.log(sumArray([])); // true
console.log(sumArray([-6, 20, -1, 10, -12])); // true

:::

### [Beginner Series #1 School Paperwork](https://www.codewars.com/kata/55f9b48403f6b87a7c0000bd)*
- 知識點：
:::spoiler 解法

function paperwork(n, m) {
  return n > 0 && m > 0 ?  n * m :  0;  
}

:::

### [Sum Arrays](https://www.codewars.com/kata/53dc54212259ed3d4f00071c)*
- 知識點：Array.prototype.reduce()
:::spoiler 解法
- 注意傳入 [] 要回傳 0 ，需要給初始值

function sum (numbers) {
    "use strict";
    return numbers.reduce((a,c)=>a+c,0);
};

:::

### [Convert a Number to a String!](https://www.codewars.com/kata/5265326f5fda8eb1160004c8)**
- 知識點：數字轉字串的各種方法
:::spoiler 解法

function numberToString(num) {
  return num.toString();
}
function numberToString(num) {
  // Return a string of the number here!
  return String(num);
}
function numberToString(num) {
  return ''+num;
}
const numberToString = num => `${num}`;

- 此程式碼旨在執行toString()（對於整數）的操作，但不使用內建函數。

function numberToString(num) {
  //create a new empty string in which the result will be written
  var str = '';
  
  //Check, whether the number is positive or negative. The multiplier 1 or -1 will be saved for later
  const mult = num < 0 ? -1 : 1;
  
  //Take the absolut value of num. Could also be done with Math.abs
  num *= mult;

  //Loop, which will run as often as num has digits
  do {
    //Take the least significant digit of num (num % 10), add 48 and get the associated ascii character. The number 0 has the ascii value of 48, 1 = 49... 9 = 57.
    //Then prepend that character to the string.
    str = String.fromCharCode(num % 10 + 48) + str;
    
    //Remove the least significant digit from num. (Divide by ten, then round down)
    num = ~~(num / 10);
  } while (num > 0);
  
  //If the input number was negative, prepend a -.
  if(mult < 0) str = "-" + str;
  return str;
}

:::

### [Sentence Smash](https://www.codewars.com/kata/53dc23c68a0c93699800041d)*
- 知識點：Array.prototype.join()
:::spoiler 解法

function smash (words) {
   return words.join(" ");
};

:::

### [Calculate average](https://www.codewars.com/kata/57a2013acf1fa5bfc4000921)****
- 知識點：Array.prototype.reduce()
:::spoiler 解法
- 要用`array.length === 0`來檢查是否為空陣列，而非`array === []`，會報錯，不能使用===運算符來比較兩個陣列是否相等

function findAverage(array) {
  if (array.length == 0) {
    return 0;
  } else {
    return array.reduce((a, c) => a + c) / array.length;
  }
}
console.log(findAverage([]));

:::

### [Even or Odd](https://www.codewars.com/kata/53da3dbb4a5168369a0000fe)*
- 知識點：%
:::spoiler 解法

function evenOrOdd(number) {
  return number % 2 === 0 ? "Even" : "Odd"
}

:::

### [Basic Mathematical Operations](https://www.codewars.com/kata/57356c55867b9b7a60000bd7)**
- 知識點：switch
:::spoiler 解法

function basicOp(operation, value1, value2)
{
    switch (operation) {
    case "+":
      return value1 + value2;
    case "-":
      return value1 - value2;
    case "*":
      return value1 * value2;
    case "/":
      return value1 / value2;
  }
}

:::

### [Thinkful - Logic Drills: Traffic light](https://www.codewars.com/kata/58649884a1659ed6cb000072)*
- 知識點：用陣列處理、三元運算子、物件解法
:::spoiler 解法

function updateLight(current) {
  const trafficLight = ["green", "yellow", "red"];
  return current === "red"
    ? trafficLight[0]
    : trafficLight[trafficLight.indexOf(current) + 1];
}
console.log(updateLight("green"));
console.log(updateLight("yellow"));
console.log(updateLight("red"));

// 物件解法
const updateLight = current => ({
  green: 'yellow',
  yellow: 'red',
  red: 'green',
})[current]

:::

### [Keep Hydrated!](https://www.codewars.com/kata/582cb0224e56e068d800003c)***
- 知識點：Math.floor()
:::spoiler 解法

function litres(time) {
  return Math.floor(time * 0.5);
}

:::

### [Remove String Spaces](https://www.codewars.com/kata/57eae20f5500ad98e50002c5)*
- 知識點：正則、[String.prototype.replace()](https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/String/replace)
:::spoiler 解法

function noSpace(x){
  return x.replace(/\s/g,"")
}

function noSpace(x){return x.split(' ').join('')}

:::

### [The Feast of Many Beasts](https://www.codewars.com/kata/5aa736a455f906981800360d)*
- 知識點：[String.prototype.slice()](https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/String/slice)為負值的話是從尾部開始算、startsWith、endsWith
:::spoiler 解法

function feast(beast, dish) {
    return beast[0] === dish[0] && beast[beast.length - 1] === dish[dish.length - 1]
}

function feast(beast, dish) {
  return beast[0]===dish[0] && beast.slice(-1)===dish.slice(-1);
}

function feast(beast, dish) {
  return dish.startsWith(beast[0]) && dish.endsWith(beast[beast.length-1])
}

:::

### [String cleaning](https://www.codewars.com/kata/57e1e61ba396b3727c000251)**
- 知識點：正則，\d 為匹配數字
:::spoiler 解法

function stringClean(s){
  return s.replace(/\d/g, "")
}

:::

### [Is the string uppercase?](https://www.codewars.com/kata/56cd44e1aa4ac7879200010b)***
- 知識點：String.prototype.match() 若沒匹配到會回傳 null
:::spoiler 解法

String.prototype.isUpperCase = function() {
  const regex = /[a-z]/g;
   return this.match(regex) === null;
 }

:::

### [Stringy Strings](https://www.codewars.com/kata/563b74ddd19a3ad462000054)**
- 知識點：String.prototype.repeat()、String.prototype.padStart()
:::spoiler 解法

function stringy(size) {
  const pair = "10";
  const pairs = pair.repeat(Math.floor(size / 2));
  return size % 2 === 1 ? pairs + 1 : pairs;
}

const stringy = size => "10".repeat(size).slice(0,size);

const stringy = x => ''.padStart(x,'10');

:::

### [Training JS #8: Conditional statement--switch](https://www.codewars.com/kata/572059afc2f4612825000d8a)**
- 知識點：switch 多 case 執行相同程式碼
:::spoiler 解法

function howManydays(month) {
  var days;
  switch (month) {
    case 2:
      days = 28;
      break;
    case 4:
    case 6:
    case 9:
    case 11:
      days = 30;
      break;
    default:
      days = 31;
  }
  return days;
}
howManydays(12);

:::

### [Will you make it?](https://www.codewars.com/kata/5861d28f124b35723e00005e)*
- 知識點：
:::spoiler 解法

const zeroFuel = (distanceToPump, mpg, fuelLeft) => {
  return distanceToPump <= mpg * fuelLeft;
};

:::

### [Counting sheep...](https://www.codewars.com/kata/54edbc7200b811e956000556)*
- 知識點：
:::spoiler 解法

function countSheeps(sheep) {
  let sheepCount = 0;
  sheep.forEach(e => e === true ? sheepCount++ : sheepCount)
  return sheepCount
}

:::

### [I love you, a little , a lot, passionately ... not at all](https://www.codewars.com/kata/57f24e6a18e9fad8eb000296)***
- 知識點：調換陣列順序
:::spoiler 解法

function howMuchILoveYou(nbPetals) {
  const phrases = [
    "I love you",
    "a little",
    "a lot",
    "passionately",
    "madly",
    "not at all",
  ];
  return phrases[nbPetals % 6 === 0 ? phrases.length - 1 : (nbPetals % 6) - 1];
}

function howMuchILoveYou(nbPetals) {
  const phrases = [
    "not at all",
    "I love you",
    "a little",
    "a lot",
    "passionately",
    "madly",
  ];
  return phrases[nbPetals % 6];
}

:::

### [Rock Paper Scissors!](https://www.codewars.com/kata/5672a98bdbdd995fad00000f)**
- 知識點：物件寫法
:::spoiler 解法

const rps = (p1, p2) => {
  if (p1 === p2) return "Draw!";
  if (p1 === "scissors") {
    if (p2 === "rock") {
      return "Player 2 won!";
    } else if (p2 === "paper") {
      return "Player 1 won!";
    }
  }
  if (p1 === "rock") {
    if (p2 === "scissors") {
      return "Player 1 won!";
    } else if (p2 === "paper") {
      return "Player 2 won!";
    }
  }
  if (p1 === "paper") {
    if (p2 === "scissors") {
      return "Player 2 won!";
    } else if (p2 === "rock") {
      return "Player 1 won!";
    }
  }
};

const rps = (p1, p2) => {
  if (p1 === p2) return "Draw!";
  var rules = {rock: "scissors", paper: "rock", scissors: "paper"};
  if (p2 === rules[p1]) {
    return "Player 1 won!";
  }
  else {
    return "Player 2 won!";
  }
};

:::

### [Return Negative](https://www.codewars.com/kata/55685cd7ad70877c23000102)*
- 知識點：
:::spoiler 解法

function makeNegative(num) {
  return num < 0 ? num : -num;
}

:::

### [Correct the mistakes of the character recognition software](https://www.codewars.com/kata/577bd026df78c19bca0002c0)***
- 知識點：
:::spoiler 解法

function correct(string) {
  const arr = string.split("");
  arr.forEach((e, i) => {
    switch (e) {
      case "5":
        arr[i] = "S";
        break;
      case "0":
        arr[i] = "O";
        break;
      case "1":
        arr[i] = "I";
        break;
    }
  });
  return arr.join("");
}
correct("51NGAP0RE");

correct = s => s.replace(/0/g,'O').replace(/1/g,'I').replace(/5/g,'S')

const corrections = {
	'5': 'S',
	'0': 'O',
  '1': 'I',
};

const correct = string => (
	string.replace(/[501]/g, character => corrections[character])
);

:::

### [Count Odd Numbers below n](https://www.codewars.com/kata/59342039eb450e39970000a6)**
- 知識點：
:::spoiler 解法

function oddCount(n){
return n%2 === 1 ? (n-1)/2 : n/2
}

const oddCount = n => Math.floor(n/2) ;

:::

### [Find Maximum and Minimum Values of a List](https://www.codewars.com/kata/577a98a6ae28071780000989)*
- 知識點：
:::spoiler 解法

var min = function(list){
    
    return Math.min(...list);
}

var max = function(list){
    
    return Math.max(...list);
}

:::

### [Count of positives / sum of negatives](https://www.codewars.com/kata/576bb71bbbcf0951d5000044)**
- 知識點：空陣列 [] 是真值
:::spoiler 解法

function countPositivesSumNegatives(input) {
  if (!input || input.length === 0) return [];
  const arr = [];
  arr[0] = input.filter((e) => e > 0).length;
  arr[1] = input.filter((e) => e < 0).reduce((a, c) => a + c, 0);
  return arr;
}
countPositivesSumNegatives([
  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, -11, -12, -13, -14, -15,
]);

:::

### [To square(root) or not to square(root)](https://www.codewars.com/kata/57f6ad55cca6e045d2000627)**
- 知識點：
:::spoiler 解法

function squareOrSquareRoot(array) {
  return array.map((e) =>
    Number.isInteger(Math.sqrt(e)) ? Math.sqrt(e) : Math.pow(e, 2)
  );
}
console.log(squareOrSquareRoot([4, 3, 9, 7, 2, 1]));

:::

### [ASCII Total](https://www.codewars.com/kata/572b6b2772a38bc1e700007a)*
- 知識點：
:::spoiler 解法

function uniTotal (string) {
  return string.split("").reduce((a,c)=> a+ c.charCodeAt(0),0);
}

:::

### [Flick Switch](https://www.codewars.com/kata/64fbfe2618692c2018ebbddb)**
- 知識點：
:::spoiler 解法

function flickSwitch(arr){
  let state = true;
  return arr.map(e=>{
    if(e === "flick"){
      state = !state;
      return state;
    }else{
      return state;
    }
  })
}

:::

### [Gravity Flip](https://www.codewars.com/kata/5f70c883e10f9e0001c89673)**
- 知識點：
:::spoiler 解法

const flip=(d, a)=>{
  switch(d){
      case "R":
      a.sort((a,b)=> a-b);
      break;
      case "L":
      a.sort((a,b)=> b-a);
      break;
  }
  return a
}

const flip = (d, a) => a.sort((x, y) => d === 'R' ? x - y : y - x);

:::

### [FIXME: Replace all dots](https://www.codewars.com/kata/596c6eb85b0f515834000049)**
- 知識點：
:::spoiler 解法

var replaceDots = function(str) {
  return str.replace(/\./g, '-');
}

:::

### [Add Length](https://www.codewars.com/kata/559d2284b5bb6799e9000047)*
- 知識點：
:::spoiler 解法

function addLength(str) {
  const arr = str.split(" ");
  return arr.map((e) => e + " " + e.length);
}

:::

### [String Templates - Bug Fixing #5](https://www.codewars.com/kata/55c90cad4b0fe31a7200001f)**
- 知識點：
:::spoiler 解法

function buildString(...template){
  return `I like ${template.join(', ')}!`;
}

:::

### [Who is going to pay for the wall?](https://www.codewars.com/kata/58bf9bd943fadb2a980000a7)**
- 知識點：
:::spoiler 解法

function whoIsPaying(name){
  if(name.length<=2) return [name];
  return [name,name[0]+name[1]];
}

:::

### [Removing Elements](https://www.codewars.com/kata/5769b3802ae6f8e4890009d2)*
- 知識點：
:::spoiler 解法

function removeEveryOther(arr){
  const resultArr =[]
  for(let i = 0; i < arr.length; i+=2){
    resultArr.push(arr[i]);
  }
  return resultArr;
}

function removeEveryOther(arr){
  return arr.filter(function(elem, index) {
    return index % 2 === 0;
  });
}

:::

### [Chuck Norris VII - True or False? (Beginner)](https://www.codewars.com/kata/570669d8cb7293a2d1001473)*
- 知識點：其他網友解法頗有趣XDD
:::spoiler 解法

function ifChuckSaysSo(){
    return !true;
}

:::

### [Merge two sorted arrays into one](https://www.codewars.com/kata/5899642f6e1b25935d000161)**
- 知識點：Set
:::spoiler 解法

function mergeArrays(arr1, arr2) {
  const arr = [];
  arr1.forEach((e) => arr.push(e));
  arr2.forEach((e) => arr.push(e));
  arr.sort((a, b) => a - b);
  const result = new Set(arr);
  return [...result];
}

function mergeArrays(arr1, arr2) {
  return Array.from(new Set(arr1.concat(arr2).sort((a,b) => (a-b))));
}

:::

### [title]()**
- 知識點：
:::spoiler 解法

:::