例1：翻转字符串

function reverseString(str) {
  var array = [];
  for(var i = 0;i<str.length; i++){
    array.push(str[i]);
  }
 var aa = array.reverse().join('');
return aa;
}

reverseString("hello");    //olleh
reverseString("Howdy");    //ydwoH
reverseString("Greetings from Earth") ;    //htraE morf sgniteerG

例2：阶乘

function factorialize(num) {
  if(num <=1 ){
    return 1;
  }else{
      return num * factorialize(num-1);
  }
}

factorialize(5);    //120
factorialize(20);    //2432902008176640000
factorialize(0);    //1

例3：回文

function palindrome(str) {
    var a =str.replace(/[\s|\.|\,|\-|\/|\\|\_|\)|\(|\:]/g,'').toLowerCase();
    var b = a.split('').reverse().join('');
    if(b === a){
        return true;  
    }else{
        return false;
    }
}
palindrome("not a palindrome");    //false
palindrome("0_0 (: /-\ :) 0-0");    //true
palindrome("1 eye for of 1 eye.");    //false
palindrome("My age is 0, 0 si ega ym.");    //true
palindrome("almostomla");    //false
palindrome("nope");    //false
palindrome("never odd or even");    //true
palindrome("A man, a plan, a canal. Panama");
palindrome("race car");

例4：找出最大的单词，并返回它的长度

function findLongestWord(str) {      
    var arrayStr = str.split(' ');
    var array = [];
    arrayStr.forEach(function(e){    
        array.push(e.length);
    });
    var reverseStr = array.sort(compare);
    return reverseStr[reverseStr.length-1];
}
function compare(value1,value2){
    if(value1 < value2){
        return -1;
    }else if(value1 > value2){
        return 1;
    }else{
        return 0;
    }
}
findLongestWord("The quick brown fox jumped over the lazy dog");    //6

例5：将首字母替换成大写

function titleCase(str) {
      var tmp = [];
    var array = [];
    str.split(' ').forEach(function(e){ 
        tmp.push(e.toLowerCase());
    });
    tmp.forEach(function(e){
        array.push(e.replace(e.charAt(0),function(s){
            return s.toUpperCase()
        }));
    });
  return array.join(' ');
}

titleCase("I'm a little tea pot");    //I'm A Little Tea Pot

例6：找到每个数组中最大的一位数，并将它返回

function largestOfFour(arr) {
      var tmp = [];
    var array = [];
    for(var i = 0; i < arr.length; i++){
        tmp.push(arr[i].sort(compare));
    }
    tmp.forEach(function(e){
        array.push(e[0]);
    });
    return array;
}

function compare(value1,value2){
    if(value1 < value2){return 1}
    else if(value1 > value2){return -1}
    else{return 0}
}
largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

例7：检查字符串结尾

function confirmEnding(str, target) {
    var tmp = [];
    tmp = str.split(' ')
    if(tmp.length === 1){
        tmp = str.split('')
    }    
    var lastWord = tmp[tmp.length-1] 
    lastWord = lastWord.split('').reverse()
    target = target.split('').reverse()
    for(var j = 0; j < target.length;j++){
        if(target[j] === lastWord[j]){}else{
            return false
        }
    }
    return true
}
confirmEnding("He has to give me a new name", "me")  //true
confirmEnding("He has to give me a new name", "na")  //false

例8：重复输出字符串

function repeat(str, num) {
    if(num <= 0){
        return ''
    }else if(num === 1){
        return str
    }else{
        return str + repeat(str,num-1)
    }    
}

repeat("abc", -2);`//''
repeat("abc", 3);    //'abcabcabc'

例9：阶段字符串

function truncate(str, num) {
    if(num <= 3){
        return str.substring(0,num)+'...'    
    }else if(num >= str.length){
        return str.substring(0,num)
    }else{
        return str.substring(0,num-3)+'...'
    }
}
truncate("A-tisket a-tasket A green and yellow basket", "A-tisket a-tasket A green and yellow basket".length + 2)    //A-tisket a-tasket A green and yellow basket

例10：猴子吃香蕉分割数组

function chunk(arr, size) {
    var array = []    
      for(var i = 0; i < Math.ceil(arr.length / size);i++){
        var start = i * size;
        var end = start +size;
        array.push(arr.slice(start,end))
    }
    return array
}
chunk(["a", "b", "c", "d"], 2);    //[["a", "b"], ["c", "d"]]

例11：截断数组

function slasher(arr, howMany) {
      return arr.splice(howMany)
}
slasher([1, 2, 3], 2);    //[3]

例12：比较字符串，如果第一个字符串中包含了第二个字符串的所有字母，则返回true，否则返回false

function mutation(arr) {
    for(var i = 0; i < arr[1].length; i++){
          if(arr[0].toLowerCase().indexOf(arr[1][i].toLowerCase()) === -1){
            return false
        }
    }    
    return true
}
mutation(["hello", "hey"]); //true
mutation(["hello", "neo"]);    //false

例13：删掉数组中的假值

function bouncer(arr) {
    var array = []
    for(var i = 0; i < arr.length; i++){
        if(arr[i]){
            array.push(arr[i])
        }
    }
    return array
}
bouncer([7, "ate", "", false, 9]);    //[7,"ate",9]

例14：摧毁数组

function destroyer(arr) {
    return [].slice.call(arguments).reduce((current,next) =>{
        return current.filter((e) => {
            return e !== next
        })
    })
}
destroyer([1, 2, 3, 1, 2, 3], 2, 3);    //[1,1]

例15：数组排序并找出元素索引

function where(arr, num) {
    arr.push(num)
    arr.sort((value1,value2) => {
        if(value1 < value2){
            return -1
        }else if(value1 > value2){
            return 1
        }else{
            return 0
        }
    })
    return    arr.indexOf(num)
}

where([40, 60], 50);    //1

例16：凯撒密码
方法1——用数组做

function rot13(str) {
    str = str.toUpperCase()
    var tmp = []
    var array = []
    var arrayEnd = []
    for(var i = 0; i < str.length; i++){
        tmp.push(str[i].charCodeAt())
    }
    tmp.forEach((e) => {
        if(e < 65||e > 90){
            array.push(e)
        }else if(e <= 77){
            array.push(e + 13)
        }else if(e <= 90){
            array.push(e - 13)
        }
    })
    array.forEach((e) => {
        arrayEnd.push(String.fromCharCode(e))
    })
    return arrayEnd.join('')
}
rot13("SERR PBQR PNZC");  //"FREE CODE CAMP"

方法2——字符串

function rot13(str) {
    str = str.toUpperCase()
    var tmp 
    var string = ''
    tmp = str.split('')        //split 拆分字符串变成数组，join 合并数组变成字符串
    tmp.forEach((e) => {
        if(e.charCodeAt() < 65||e.charCodeAt() > 90){
            string += e
        }else if(e.charCodeAt() <= 77){
            string += String.fromCharCode(e.charCodeAt() + 13)
        }else if(e.charCodeAt() <= 90){
            string += String.fromCharCode(e.charCodeAt() - 13)
        }
    })
    return string
}
rot13("SERR PBQR PNZC");     //"FREE CODE CAMP"