# Basic Algorithm Scripting

Basic Algorithm Scripting

## Get Set for our Algorithm Challenges

* 用學過的概念回答開放式問題，練習程序猿思維
* 題目會很難，不要沮喪
* 無法招架就用 Read-Search-Ask 大招
* 準備好就開始吧

## Reverse a String

```javascript
function reverseString(str) {
  str=str.split(""); // str 不會變
  str=str.reverse(); // str 會變化
  str=str.join("");  // str 不會變
  return str;
}

reverseString("hello");
```

官方解答

* [freeCodeCamp Algorithm Challenge Guide: Reverse a String - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-reverse-a-string/16043/10)

```javascript
function reverseString(str) {
  return str.split('').reverse().join('');
}
```

## Factorialize a Number

```javascript
function factorialize(num) {
  var temp=1;

  for(i=num; i>0; i--){
    temp=temp*i;
  }
  return temp;
}

factorialize(5);
```

官方解答

* 用遞歸，函數自己呼叫自己
* [freeCodeCamp Algorithm Challenge Guide: Factorialize A Number - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-factorialize-a-number/16013/33)

```javascript
function factorialize(num) {
  if (num === 0) { return 1; }
  return num * factorialize(num-1);
}

factorialize(5);
```

## Check for Palindromes

* 官方解答有三種版本，我的解法偏向第二種
* [freeCodeCamp Algorithm Challenge Guide: Check for Palindromes - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-check-for-palindromes/16004/11)
* [正規表達式 - JavaScript | MDN](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Guide/Regular_Expressions)
* [Check for Palindromes | freeCodeCamp](https://www.freecodecamp.org/challenges/check-for-palindromes)
* [Online regex tester and debugger: PHP, PCRE, Python, Golang and JavaScript](https://regex101.com/)

```javascript
function palindrome(str) {
  // Good luck!

  // 切換成小寫，排除非字母符號，將字串轉成矩陣
  regular = /[A-Za-z0-9]/g;

  var lowerStr = str.toLowerCase();      // 轉成小寫
  var matchStr = lowerStr.match(regular);

  // 比較 i, matcherStr.length -i-1 的字母是否相同
  for (i=0; i<matchStr.length/2; i++){
    if (matchStr[i]!=matchStr[matchStr.length-1-i]){
      return false;
    }
  }

  return true;
}
palindrome("0_0 (: /-\ :) 0-0");
```

## Find the Longest Word in a String

* 官方解答的第二三種作法，對我來說有點不直覺
* 二用了`reduce()`和`math.max()`，三用了recursiveness遞歸
* [freeCodeCamp Algorithm Challenge Guide: Find the Longest Word in a String - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-find-the-longest-word-in-a-string/16015/13)
* [從陣列計算最大和最小值](http://www.jstips.co/zh_tw/javascript/calculate-the-max-min-value-from-an-array/)

```javascript
function findLongestWord(str) {

  var strArray = str.split(" ");
  var strLength=0;

  for(i=0; i<strArray.length; i++){
    if(strArray[i].length>strLength){
      strLength = strArray[i].length;
      console.log("i="+i);
      console.log("strLength="+strLength);
    }
  }

  return strLength;

}

findLongestWord("The quick brown fox jumped over the lazy dog");
```

我的第二種，用max

```javascript
function findLongestWord(str) {
  // 请把你的代码写在这里

  strArray = str.split(" ");
  numArray = [];

  for(i=0; i<strArray.length; i++){
    numArray[i]=strArray[i].length;
  }

  biggestLength = Math.max(...numArray);

  return biggestLength;
}

findLongestWord("The quick brown fox jumped over the lazy dog");
```

## Title Case a Sentence

* 想這題，覺得腦袋抽筋
* 官方有三種解法
* 一，用 `substr()` 取出字串，處理後合併
* 二，`map()`循環，直接用`replace()`取代，`charAt()`抓出字串
* 三，`replace()`取代，並用正則表達式
* [freeCodeCamp Algorithm Challenge Guide: Title Case a Sentence - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-title-case-a-sentence/16088)

```javascript
return str.toLowerCase()
      .replace(/(^|\s)\S/g, (L) => L.toUpperCase());`
```

自己的解法，第一種

```javascript
function titleCase(str) {

  // 轉小寫，將各單字存進矩陣
  LowerStr = str.toLowerCase();
  strArray = LowerStr.split(" ");
  wordArray =[];

  // 把每個字母存進矩陣，並首字轉大寫
  for(i=0; i<strArray.length; i++){
    for(j=0; j<strArray[i].length; j++){
      wordArray[i]=strArray[i].split("");
    }
    wordArray[i][0] = wordArray[i][0].toUpperCase();
  }

  // 將字母合併回單字
  for(i=0; i<wordArray.length; i++){
    wordArray[i] = wordArray[i].join("");
  }

  // 將單字合併回句子
  UpperStr = wordArray.join(" ");

  return UpperStr;
}

titleCase("I'm a little tea pot");
```

自己的解法，第二種

* [JavaScript replace() 方法](http://www.w3school.com.cn/jsref/jsref_replace.asp)
* [JavaScript substring() 方法](http://www.w3school.com.cn/jsref/jsref_substring.asp)
* [JavaScript slice() 方法](http://www.w3school.com.cn/jsref/jsref_slice_string.asp)

```javascript
function titleCase(str) {
  // 请把你的代码写在这里

  // 小寫.分詞
  strArray = str.toLowerCase().split(" ");

  // 取首字大寫，連接其他字
    for(i=0; i<strArray.length; i++){
      strArray[i] = strArray[i].replace(strArray[i].charAt(0), strArray[i].charAt(0).toUpperCase());
    }

    strJoin = strArray.join(" ");

  console.log(strJoin);


  return strJoin;
}

titleCase("I'm a little tea pot");
```

自己的解法，第三種

* [Array.prototype.map() - JavaScript | MDN](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Reference/Global_Objects/Array/map)

```javascript
function titleCase(str) {
  // 请把你的代码写在这里

  // 小寫.分詞
  strArray = str.toLowerCase().split(" ");

  // 取首字大寫
  upStrArray = strArray.map(function (val){
      return val.replace(val.charAt(0),val.charAt(0).toUpperCase());
  });

    strJoin = upStrArray.join(" ");

  console.log(strJoin);


  return strJoin;
}

titleCase("I'm a little tea pot");
```

自己的解法，第四種

* 前面 `split()` 分詞的目的，是為了提取第一個字以便改成大寫。但用這則表達式的話，就能直接抓取每個字的首字
* 箭頭函數如果只執行一行，就不用加 return&#x20;
* [Regular Expression (RegExp) in JavaScript - 石頭閒語](http://rocksaying.tw/archives/2670695.html)
* [KingKong Bruce記事: JavaScript - 表單元素 - Regular Expression](http://blog.kkbruce.net/2012/02/javascript-regular-expression.html#.WXr0pnUjFpg)

```javascript
function titleCase(str) {
  // 请把你的代码写在这里

  // 取首字大寫
  lowStr = str.toLowerCase();
  var upStr = str.replace(/(^|\s)\S/g, (L) => L.toUpperCase());

  console.log(upStr);

  return upStr;
}

titleCase("I'm a little tea pot");
```

自己的解法，第五種

* 匿名函數要用 return 回傳

```javascript
function titleCase(str) {
  // 请把你的代码写在这里

  // 取首字大寫
  lowStr = str.toLowerCase();
  var upStr = lowStr.replace(/(^|\s)\S/g, function(L){
      return L.toUpperCase();
    });

  console.log(upStr);

  return upStr;
}

titleCase("I'm a little tea pot");
```

## Return Largest Numbers in Arrays

* [freeCodeCamp Algorithm Challenge Guide: Return largest Numbers in Arrays - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-return-largest-numbers-in-arrays/16042)

我自己的解法

* 要先宣告變數為矩陣，不然會有問題
* [javascript - Uncaught TypeError: Cannot set property '0' of undefined " - Stack Overflow](https://stackoverflow.com/questions/10673237/uncaught-typeerror-cannot-set-property-0-of-undefined)

```javascript
function largestOfFour(arr) {
  // You can do this!
  var bigArr=[];

  for(i=0; i<arr.length; i++){
      var temp=0;
      for(j=0; j<arr[i].length; j++){
          if(arr[i][j]>temp){
            temp=arr[i][j];
          }
      }
      bigArr[i]=temp;
  }

  console.log(bigArr);
  return bigArr;
}


largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);
```

討論區看到的一個好懂的解法

* `Math.max`不吃矩陣，所以要加.apply
* [從陣列計算最大和最小值](http://www.jstips.co/zh_tw/javascript/calculate-the-max-min-value-from-an-array/)
* [javascript - How does the Math.max.apply() work? - Stack Overflow](https://stackoverflow.com/questions/21255138/how-does-the-math-max-apply-work)

```javascript
function largestOfFour(arr) {
  return arr.map(function(a) {
      return Math.max.apply(null, a);
  });
}

 largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);
```

## Confirm the Ending

* [freeCodeCamp Algorithm Challenge Guide: Confirm the Ending - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-confirm-the-ending/16006)
* [String.prototype.endsWith() - JavaScript | MDN](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Reference/Global_Objects/String/endsWith)
* [相等比較 - JavaScript | MDN](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Equality_comparisons_and_sameness)

我的解答

```javascript
function confirmEnding(str, target) {
  // "Never give up and good luck will find you."
  // -- Falcor

  var strEnd = str.substring(str.length - target.length, str.length);

  return strEnd == target;
}

confirmEnding("Bastian", "n");
```

官方解答

```javascript
function confirmEnding(str, target) {
  return str.substr(-target.length) === target;
}
```

## Repeat a string repeat a string

* push 進陣列再 join 成字串的作法好蠢阿阿阿阿阿
* 有人說用 while 比較省時間
* concat
* repeat
* 遞迴
* [freeCodeCamp Algorithm Challenge Guide: Repeat a string repeat a string - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-repeat-a-string-repeat-a-string/16041)

```javascript
function repeatStringNumTimes(str, num) {
  // repeat after me
  var timeStr=[];
  for(i=0; i<num; i++){
    timesStr=timeStr.push(str);
  }
  return timeStr.join("");
}

repeatStringNumTimes("abc", 3);
```

## Truncate a string

* [freeCodeCamp Algorithm Challenge Guide: Truncate a String - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-truncate-a-string/16089/19)
* [JavaScript取子串方法slice,substr,substring对比表 - Ider - 博客园](http://www.cnblogs.com/ider/p/js-slice-vs-substr-vs-substring-table.html)

自己想的方法

```javascript
function truncateString(str, num) {
  // Clear out that junk in your trunk
  var dotstr;

  if(num>=str.length){
      dotstr = str;
  } 
  else if(num<3){
      dotstr = str.slice(0,num)+"...";
  }
  else if(num<str.length){
      dotstr = str.slice(0,num-3)+"...";
  }

  console.log(dotstr);
  return dotstr;
}

truncateString("A-tisket a-tasket A green and yellow basket", 11);
```

官方的進階解答

```javascript
function truncateString(str, num) {
  if (str.length <= num) {
    return str;
  } else {
    return str.slice(0, num > 3 ? num - 3 : num) + '...';
  }
```

## Chunky Monkey

* [freeCodeCamp Algorithm Challenge Guide: Chunky Monkey - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-chunky-monkey/16005)
* [Array.prototype.slice() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice)
* [Array.prototype.splice() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice)

自己想到的作法

```javascript
function chunkArrayInGroups(arr, size) {
  // Break it up.
  var sliceArr = [];

  for(i=0; i<arr.length/size; i++){
      sliceArr.push(arr.slice(i*size, (i+1)*size));
  }

  return sliceArr;

}

chunkArrayInGroups([0, 1, 2, 3, 4, 5, 6, 7, 8], 2);
a= chunkArrayInGroups([0, 1, 2, 3, 4, 5, 6, 7, 8], 2);
console.log(a);
```

官方進階的解法

* [Array.prototype.splice() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice)

```javascript
 function chunk(arr, size) {
  // 请把你的代码写在这里
  var spliceArr = [];

  while(arr.length){
    spliceArr.push(arr.splice(0,size));
  }

  return spliceArr;
}

chunk(["a", "b", "c", "d", 1, 2, 3, 4, 5], 2);
```

## Slasher Flick

痾，數組截斷者

* 這題會不會有點簡單，雖然標題我完全看不懂什麼意思
* [freeCodeCamp Algorithm Challenge Guide: Slasher Flick - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-slasher-flick/16047)

```javascript
function slasher(arr, howMany) {
  // it doesn't always pay to be first
  return arr.splice(howMany, arr.length);
}

slasher([1, 2, 3], 2);
```

## Mutations

判斷arr\[1]的字母是不是都在arr\[0]出現過

* [freeCodeCamp Algorithm Challenge Guide: Mutations - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-mutations/16025)
* [String.prototype.indexOf() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/indexOf)

我的作法

```javascript
function mutation(arr) {
  var splitSec = arr[1].toLowerCase().split("");
  var pos = 0;

  for(i=0; i<splitSec.length; i++){
      if(pos!=-1){
          pos = arr[0].toLowerCase().indexOf(splitSec[i]);
          console.log(pos);
          if(pos==-1){
              return false;
          }
      }
      else if(pos==-1){
          return false;
      }
  }

  return true;
}

mutation(["hello", "hey"]);
```

第二種

```javascript
function mutation(arr) {
  // 请把你的代码写在这里

  one = arr[0].toLowerCase();
  two = arr[1].toLowerCase();

  for(i=0; i<two.length; i++){
      if(one.indexOf(two[i]) === -1 ){
          return false;
      }
  }

  return true;
}

a = mutation(["hello", "he"]);
console.log(a)
```

* [JavaScript Array.prototype.every() - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/javascript-array-prototype-every/14287)

先練習 every 的用法

```javascript
var arr = [true, false, true];

function test(arr){
    return arr.every(function(val){
        return val === true;
    })
}

console.log(test(arr)); // false
```

自己的寫法

```javascript
function mutation(arr) {
  // 请把你的代码写在这里

  // 先定義變數，把arr[0]轉成小寫，把arr[1]轉成小寫再分割字母
  var FirStr = arr[0].toLowerCase();
  var SecArr = arr[1].toLowerCase().split("");

  // SecArr 的字母，在FirStr中都有位置(indexOf回傳不是-1)?
  result = SecArr.every( (val) => FirStr.indexOf(val) !== -1 );

  //回傳結果
  console.log(result);
  return result;
}

mutation(["hello", "h"]);
```

## Falsy Bouncer

* 去除 falsy 值，也就是 boolean 值為 false 的值
* [freeCodeCamp Algorithm Challenge Guide: Falsy Bouncer - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-falsy-bouncer/16014/41)
* [Boolean - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean)
* [Array.prototype.filter() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter)

```javascript
function bouncer(arr) {
  // Don't show a false ID to this bouncer.

  return arr.filter( (val) => Boolean(val) != false );

}

bouncer([7, "ate", "", false, 9]);
```

留言串看到的解法

```javascript
function bouncer(arr) {
  var a = [];
  arr.forEach(function(el){
  if(el){
    a.push(el);}});
 return a;}
```

列舉式的寫法

```javascript
function badValues(val){  
 return val !== false && val !== null && val !== 0 && val !== "" && val !== undefined && !Number.isNaN(val);   
}

function bouncer(arr) {
  return arr.filter(badValues);  
}

bouncer([1, null, NaN, 2, undefined]);
```

## Seek and Destroy

從矩陣中移除後面參數出現的東西

* 參數物件，第一次聽到這個東西
* [Arguments object - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments)
* [freeCodeCamp Algorithm Challenge Guide: Seek and Destroy - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-seek-and-destroy/16046)

```javascript
function destroyer(arr) {
  // Remove all the values

  // 先拿到參數矩陣，再另存參數一矩陣
  var args = Array.from(arguments);
  var arr0 = args.shift(); //arr0等於參數一，args等於其他的參數

  // 排除arr0 中含有 agrs 的內容
  // callback 函數有三個參數，當前值，目錄，與要篩選的矩陣
  // 如果args[i]==val，回傳 false，代表從arr0篩除
   return arr0.filter(function(val, index, arr0){

    var result = true;

    // 如果有一個
    for(i=0; i<args.length; i++){
       if (args[i]===val){
         result = false;
       }
     }
     return result;

   });
}

a = destroyer([1, 2, 3, 1, 2, 3], 2, 3);
console.log(a)
```

其他解法

```javascript
var args = Array.prototype.slice.call(arguments, 1);

return arr.filter(function(val){ return args.every(x => x !== val); });
```

其他解法

```javascript
function destroyer(arr) {
    var filterList = [];

    for (var i = 1; i < arguments.length; i++) {
        filterList.push(arguments[i]);
    }

    return arr.filter(function(val) {
        return filterList.indexOf(val) < 0;
    });
}

destroyer([1, 2, 3, 1, 2, 3], 2, 3);
```

其他解法

```javascript
function destroyer(arr) {
 var args = Array.prototype.slice.call(arguments, destroyer.length); 

  return arr.reduce((acc, item) => {
         if (!args.includes(item)) { 
            acc.push(item);
         }  
         return acc;

 }, []);
```

官方解法一

```javascript
function destroyer(arr) {
  var args = Array.prototype.slice.call(arguments);

  for (var i = 0; i < arr.length; i++) {
    for (var j = 0; j < args.length; j++) {
      if (arr[i] === args[j]) {
        delete arr[i];
      }
    }
  }
  return arr.filter(Boolean);
```

官方解法二

```javascript
function destroyer(arr) {
  var args = Array.from(arguments).slice(1);
  return arr.filter(function(val) {
    return !args.includes(val);
  });
}
```

## Where do I belong

* [freeCodeCamp Algorithm Challenge Guide: Where do I Belong - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-where-do-i-belong/16094)

我的解法

* 一開始忘記 sort 的特性，做了許多白工
* [Array.prototype.sort() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort)

```javascript
function getIndexToIns(arr, num) {
  // Find my place in this sorted array.

  arr.push(num);
  arr.sort((a,b)=>a-b);

  return arr.indexOf(num);

}

getIndexToIns([2, 5, 10], 15);
```

官方的中等作法

* [Array.prototype.findIndex() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex)

```javascript
function getIndexToIns(arr, num) {
  // sort and find right index
  var index = arr.sort((curr, next) => curr > next)
    .findIndex((currNum)=> num <= currNum);
  // Returns proper answer
  return index === -1 ? arr.length : index;
}

getIndexToIns([40, 60], 500);
```

官方的進階作法

* [Method Chaining in JavaScript | Gregory Schier](https://schier.co/blog/2013/11/14/method-chaining-in-javascript.html)
* [Array.prototype.concat() - JavaScript | MDN](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/concat?v=example)

```javascript
function getIndexToIns(arr, num) {

return arr.concat(num).sort((a,b) => a-b).indexOf(num);

}

getIndexToIns([1,3,4],2);
```

## Caesars Cipher

* [freeCodeCamp Algorithm Challenge Guide: Caesars Cipher - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/freecodecamp-algorithm-challenge-guide-caesars-cipher/16003)
* [正規表達式 - JavaScript | MDN](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Guide/Regular_Expressions)
* [String.prototype.charCodeAt() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/charCodeAt)
* [String.fromCharCode() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/fromCharCode)

```javascript
function rot13(str) { // LBH QVQ VG!
  var result="";
  var zAt = "Z".charCodeAt(0);
  var aAt = "A".charCodeAt(0);

  for(i=0; i<str.length; i++){
    if(/\W/.exec(str[i])){
      result = result + str[i];
    }
    else if( (zAt - str.charCodeAt(i)) < 13){
      result = result + String.fromCharCode(str.charCodeAt(i)-13);
    }
    else{
      result = result + String.fromCharCode(str.charCodeAt(i)+13);
    }  
  }

  console.log(result);
  return result;
}

// Change the inputs below to test
rot13("SERR CVMMN!");
```

官方解答有三種解法

官方解法一

* `arr.map.call(str, callback)` 用在非 array 的物件(arr本身不重要)
* [Array.prototype.map() - JavaScript | MDN](https://developer.mozilla.org/zh-TW/docs/Web/JavaScript/Reference/Global_Objects/Array/map)

```javascript
function rot13(str) {
  // Split str into a character array
  return str.split('')
  // Iterate over each character in the array
    .map.call(str, function(char) {
      // Convert char to a character code
      x = char.charCodeAt(0);
      // Checks if character lies between A-Z
      if (x < 65 || x > 90) {
        return String.fromCharCode(x);  // Return un-converted character
      }
      //N = ASCII 78, if the character code is less than 78, shift forward 13 places
      else if (x < 78) {
        return String.fromCharCode(x + 13);
      }
      // Otherwise shift the character 13 places backward
      return String.fromCharCode(x - 13);
    }).join('');  // Rejoin the array into a string
}
```

官方解法二

* `fn.apply(something, array)` 可以把矩陣的內容當作參數個別傳入
* [深入浅出妙用 Javascript 中 apply、call、bind - WEB前端 - 伯乐在线](http://web.jobbole.com/83642/)
* [【javascript】call 和 apply « 和平，奮鬥，救WEB](http://fireqqtw.logdown.com/posts/258035-javascriptcall-and-apply)
* [javascript - How does the Math.max.apply() work? - Stack Overflow](https://stackoverflow.com/questions/21255138/how-does-the-math-max-apply-work)
* regular.test() 傳回 boolean
* [RegExp.prototype.test() - JavaScript | MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/test)

```javascript
// Solution with Regular expression and Array of ASCII character codes
function rot13(str) {
  var rotCharArray = [];
  var regEx = /[A-Z]/ ;
  str = str.split("");
  for (var x in str) {
    if (regEx.test(str[x])) {
      // A more general approach
      // possible because of modular arithmetic
      // and cyclic nature of rot13 transform
      rotCharArray.push((str[x].charCodeAt() - 65 + 13) % 26 + 65);
    } else {
      rotCharArray.push(str[x].charCodeAt());
    }
  }
  str = String.fromCharCode.apply(String, rotCharArray);
  return str;
}

// Change the inputs below to test
rot13("LBH QVQ VG!");
```

第三種解法

* $0 代表正則表達式篩出來的內容，$1 代表內容的第一個被篩出來的部份
* [JavaScript String.prototype.replace() - Guide - freeCodeCamp Forum](https://forum.freecodecamp.org/t/javascript-string-prototype-replace/15942)
* [javascript - $0 and $1 in Regular Expression - Stack Overflow](https://stackoverflow.com/questions/18689032/0-and-1-in-regular-expression)
* [\[ JS 进阶 \] test, exec, match, replace - kraaas前端博客 - SegmentFault](https://segmentfault.com/a/1190000003497780)

```javascript
function rot13(str) { // LBH QVQ VG!
  return str.replace(/[A-Z]/g, L => String.fromCharCode((L.charCodeAt(0) % 26) + 65));
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ayugioh2003.gitbook.io/free-code-camp-practice/front-end-development-certification/basic-algorithm-scripting.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.