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 v
4000
al.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 morn
console.log(str3); // OUTPUT: morning is upon u
console.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，
ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm

[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 test
rot13("LBH QVQ VG");
//"YOU DID IT"