freecodecamp JavaScript学习(四)

Reverse a String

function reverseString(str) {  //covert string to an array   //var strArray=str.split('');  //reverse the new array   //strArray.reverse();  //join the array to a string  //return strArray.join('');  return str.split('').reverse().join('');}

Check for Palindromes

function palindrome(str) {  // Good luck!  var filteredStr;  filteredStr=str.toLowerCase().split ('').filter(function(val){    return (val>='a' && val<='z') || (val >='0' && val<='9');  }).join('');  if (filteredStr===filteredStr.split('').reverse().join('')){    return true;  }  return false;}

Find the Longest Word in a String

function findLongestWord(str) {  //convert string to array  var arr=str.split(' ');  var arrLen=[];  for(var i=0;i<arr.length;i++){    arrLen.push({"name":arr[i],"strLen":arr[i].length});  }  arrLen.sort(function(a,b){    return b.strLen-a.strLen;  });  return arrLen[0].strLen;}

Title Case a Sentence

function titleCase(str) {  var arr=str.split(' ');  var newArr;  newArr=arr.map(function(val,index){    return val.substr(0,1).toUpperCase() + val.substr(1).toLowerCase();  });  return newArr.join(' ');}titleCase("I'm a little tea pot");//I'm A Little Tea Pot

Return Largest Numbers in Arrays

function largestOfFour(arr) {  // You can do this!  var newArr=[];  for(var i=0;i<arr.length;i++){    arr[i].sort(function(a,b){      return b-a;    });    newArr.push(arr[i][0]);  }  return newArr;}largestOfFour([[4, 9, 1, 3], [13, 35, 18, 26], [32, 35, 97, 39], [1000000, 1001, 857, 1]]);//[9,35,97,1000000]

Repeat a string repeat a string

function repeatStringNumTimes(str, num) {  // repeat after me  var repeatStr;  if(Number.isInteger(num) && num>0){    repeatStr=str.repeat(num);    return repeatStr;  }  return '';}repeatStringNumTimes("abc", 3);//"abcabcabc"

Truncate a string

MDN上的slice

function truncateString(str, num) {  // Clear out that junk in your trunk  var newStr;  if(str.length > num){    if (num>3){      newStr = str.slice(0,num-3) + '...';      return newStr;    }else{      newStr = str.slice(0,num-str.length) + '...';      return newStr;          }  }  return str;}truncateString("Absolutely Longer", 2);//Ab...truncateString("A-tisket a-tasket A green and yellow basket", 11)

var str1 = 'The morning is upon us.', // the length of str1 is 23.    str2 = str1.slice(1, 8),    str3 = str1.slice(4, -2),    str4 = str1.slice(12),    str5 = str1.slice(30);console.log(str2); // OUTPUT: he mornconsole.log(str3); // OUTPUT: morning is upon uconsole.log(str4); // OUTPUT: is upon us.console.log(str5); // OUTPUT: ""

var str = 'The morning is upon us.';str.slice(-3);     // returns 'us.'str.slice(-3, -1); // returns 'us'str.slice(0, -1);  // returns 'The morning is upon us'

Chunky Monkey

function chunkArrayInGroups(arr, size) {  // Break it up.  var newArr=[];  var tempArr=[];  for(var i=0;i<arr.length;i++){    tempArr.push(arr[i]);    if(((i+1)%size===0) || ((i+1)===arr.length)){      newArr.push(tempArr);      tempArr=[];    }  }  return newArr;}chunkArrayInGroups(["add11", "b", "c", "d", "dddee"], 3);//[["add11","b","c"],["d",, "dddee"]]

Slasher Flick

MDN上的splice

array.splice(start)array.splice(start, deleteCount)array.splice(start, deleteCount, item1, item2, ...)

MDN上的slice
-array.splice会改变原来的数组，删掉从start开始的deleteCount个数的元素，item1,item2会加在start这个位置

arr.slice()arr.slice(begin)arr.slice(begin, end)

-array.slice不改变原来的数组，会产生一个新的数组，删除begin到end-1的元素

Mutations

MDN上的string.indexOf
-查询字符串是否在当前字符串中存在。如存在，返回位置，不存在，返回-1

function mutation(arr) {  if (arr.length>=2){    var mapStr=arr[0];    var checkStr=arr[1];    for(var i=0;i<checkStr.length;i++){      if(mapStr.toLowerCase().indexOf(checkStr.toLowerCase().substr(i,1)) === -1){        return false;      }        }    return true;  }  return false;}mutation(["hello", "hey"]);// should return false.mutation(["hello", "Hello"]) //should return true

Falsy Bouncer

MDN上的Boolean
-Boolean(val)，若参数为false, null, 0, “”, undefined, and NaN，则返回false
-任何对象作为输入参数，只要对象的值不为undefined or null,则返回true

function bouncer(arr) {  // Don't show a false ID to this bouncer.  var nwArr=arr.filter(function(val){    var falsyOjb=Boolean(val);    return falsyOjb;  });  return nwArr;}bouncer([7,NaN, "ate", "", false,undefined,0,-0, 9]);//[7,"ate",9]

Seek and Destroy

1）命名参数只有一个不代表此函数只有一个参数，调用函数时不必和命名参数一致，arguments.length内属性可以获知有多少个参数传递给了函数
2）arguments是一个类似数组，不是一个真正的数组，只有一个属性length，使用Array.from将其转换为一个真正的数组
3）MDN上的arguments

function destroyer(arr) {  var newArr=[];//必须在arr.filter外面将arguments赋值给另外的变量，否则在arr.filter内使用的arguments将是filter内函数本身的参数（val）  var args = Array.from(arguments); //检测第一个参数是否为数组  if(Array.isArray(arr)){    newArr=arr.filter(function(val){      var flag=true;       for(var i=1;i<args.length;i++){         if(val==args[i]){           flag=false;           break;         }       }        return flag;    });    return newArr;  }  return "The first argument is not a array!";}destroyer([1, 2, 3, 1, 2, 3], 2, 3);//[1，1]

Caesars Cipher

-ROT13（回转13位)密码，ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyzNOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm[MDN:charCodeAt](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/charCodeAt)-str.charCodeAt(index)，得到string中参数位置的ascII码[MDN:fromCharCode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/fromCharCode)-String.fromCharCode(num1[, ...[, numN]])，num为ascii码，从ascii码得到字符function rot13(str) { // LBH QVQ VG!  var strArr=[]; //iterate the str, and get every ascii code  for(var i=0;i<str.length;i++){    if(str.substr(i,1)>='A' && str.substr(i,1)<='M'){      strArr.push(String.fromCharCode(str.charCodeAt(i)+13));    }else if(str.substr(i,1)>='N' && str.substr(i,1)<='Z'){      strArr.push(String.fromCharCode(str.charCodeAt(i)-13));    }    else{      strArr.push(str.substr(i,1));    }  } //get the string from array    return strArr.join('');}// Change the inputs below to testrot13("LBH QVQ VG");//"YOU DID IT"