preface

Common built-in objects and methods include Array, Math, String, and date object methods.

Array Array objects

Push () adds one or more elements to the end of the array and returns the new length of the array

Arr = [1, 2, 3] arr. Push (" a ", "b"); // Insert string arr.push(6); // insert a new element at the end of the array arr.push(6,7,8); // insert new elements at the end of the array a=arr.push(6,7,8); // Return the new length of the array console.log(arr) by adding it to the tail element; console.log(a);Copy the code

Returns the new length of the array

Unshift () adds one or more elements to the array header and returns the new array length

Arr = [1, 2, 3] arr. Unshift (0); // Insert a new element at the head of the array arr.unshift(-3,-2,-1,0); Var a=arr.unshift(-3,-2,-1,0); // Return the new length of the array console.log(arr); console.log(a);Copy the code

Returns the new length of the array

Pop () removes the last element at the end of the array and returns the deleted element

Arr = [6] arr. Pop (); Var a=arr.pop(); //pop removes the last element of the array and returns the deleted element to console.log(arr); console.log(a);Copy the code

Return the deleted element

Shift () removes the first element of the array and returns the deleted element

Arr = [6] arr. The shift (); Var a=arr.shift(); // Delete the first element of the array and return the deleted element console.log(arr); console.log(a);Copy the code

Returns the deleted element

Deleting or adding changes the length of the array

5, concat() array merge, return a new array, the original two arrays are not changed

Var arr = [1, 2, 3, 4]; Var arr1 =,6,7,8 [5]; var arr2=arr.concat(arr1); Console. log(arr2); console.log(arr2); console.log(arr2); var arr3=arr.concat(0,-1,-2); Console. log(arr3) var arr4=arr.concat(0,arr1,["A","B"]); Console. log(arr4) var arr5=arr.concat(); Console. log(arr5) console.log(arr5)Copy the code

6. Join () returns each element of the array as a new string concatenated with the specified character

Var arr = [1, 2, 3, 4, 5]; Var STR =arr.join(); // Merge arrays into strings, using commas by default, and connect console.log(STR); var str=arr.join("|"); // In this case, the join argument is the string concatenation v console.log(STR); var str=arr.join(""); // "" as a concatenator, will concatenate the array elements end to end into the string console.log(STR);Copy the code

7. Splice () this method removes a given number of elements from a specified location, inserts the desired elements at that location, and returns a new array of the deleted elements

Splice (where to start, how many elements to delete, elements to insert);

1) splice deletes all array return values

var arr1=arr.splice(); // Returns an empty array console.log(arr1) with no arguments; Var arr1=arr.splice(0); var arr1=arr.splice(0); // Move all data to a new array console.log(arr1);Copy the code

2) Splice can be used from front to back or from back to front

Var arr1 = arr. Splice (0, 3); Arr1 var arr1=arr.splice(-2); Var arr1=arr.splice(0,1,-1); var arr1=arr.splice(0,1,-1); Var arr1=arr.splice(-1,1,0); var arr1=arr.splice(-1,1,0); Var arr1=arr.splice(2,2,10,11); var arr1=arr.splice(2,2,10,11); // replace the first two elements of the array with 10,11Copy the code

3) Splice can insert one or more elements

Var arr = [1, 2, 3, 4, 5]; Arr. Splice (2, 0, 1); // Insert an element in the second part of the array -1 console.log(arr); Arr. Splice (2, 0, 1, 2, 3, 4); // Insert multiple elements console.log(arr) in the second part of the array;Copy the code

8. Slice (start,end) intercepts the contents of the copied array at the specified location and returns the new array without changing the original array

Starting with the subscript start, intercepts to end, including start but not end. The second parameter is not written, and is truncated to the tail by default, only from front to back

Var arr1 = arr. Slice (1, 4); Var arr1=arr.slice(); // Copy array arr var arr1=arr.slice(0); Var arr1=arr.slice(3); Var arr1=arr.slice(-2); Var arr1=arr.slice(-3,-1); // From third to lastCopy the code

9, The indexOf position is the subindex

If the index of the element is not found, -1 is returned. If the index of the element is not found, -1 is returned

1) Objects cannot be queried

var arr=[{a:1},{a:2},{a:3},{a:4}]; var index=arr.indexOf({a:1}); Console. log(index); console.log(index);Copy the code

If no query is found, -1 is returned

2) Query all the same elements

Var arr =,3,1,2,3,5,2,3,4,6 [1]; var index=arr.indexOf(3); // Only the first 3 (subscript 1) console.log(index) can be queried; Var index = arr. IndexOf (3, 2); // Only the second 3 (subscript 4) is console.log(index);Copy the code

Query all the same elements using a circular query

Var arr =,3,1,2,3,5,2,3,4,6 [1]; var index=0; While (true){index= arr.indexof (3,index); while(true){index= arr.indexof (3,index); console.log(index); if(index===-1) break; // select * from index++; // select * from index++; }Copy the code

10, Array.from(array-like list) to Array

1) Obtain the tag list according to the tag name (obtain the tag list, not array, can not directly use the array method

var divs=document.getElementsByTagName("div"); Divs.pop (); // Get the HTML element whose tag name is div; Var arr= array. from(divs); / / the ES6 method and the divs this list, into an Array / / ES5 writing = > var arr = Array. The prototype. Slice. Call (divs);Copy the code

2) Add click events to all div elements

var divs=document.getElementsByTagName("div"); var arr=Array.from(divs); For (var I =0; i<arr.length; Arr [I]. Onclick =clickHandler; } function clickHandler(){ console.log(this); var index=arr.indexOf(this); console.log(index); // If you click on the element in the list}Copy the code

11, The lastIndexOf(where to start the search) searches from back to front

Var arr =,3,1,2,3,5,2,3,4,6 [1]; var index=arr.lastIndexOf(3); console.log(index); // Print the subscript 7Copy the code

Traversal groups (forEach and Map)

1) forEach

Arr. ForEach (function(element in array, subscript of each element, array itself){})

Var arr =,2,3,5,6,7,8,9 [1]; arr.forEach(function(a,b,c){ console.log(a,b,c); })Copy the code

ForEach does not return a value

2) Map returns a new array of the same length as the original array

arr.map(function(item,index,arr){ })

Var arr =,5,7,9,1,2,4 [3]; var arr2=arr.map(function(item,index,arr){ // console.log(item,index,arr); // return "a"; return item+10; });});}); console.log(arr2);Copy the code

Var arr=[1,3,5,7,2,4,6,8]; var arr1=arr.map(function(item){ if(item>4){ return item; }}); console.log(arr1);Copy the code

Map returns a value, a new array of the same length as the original array, and the contents of the elements are determined by return

13, arr.sort() sort only numbers up to 10

Cons: Sort by character sort(function(last, last){}) only applies to numeric values

1) Numerical sorting

Var arr =,3,5,7,2,4,6,8 [1]; arr.sort(function(a,b){ return a-b; // return b-a; // from large to small}) console.log(arr);Copy the code

2) Character sorting, first convert characters to Unicode encoding

var arr=["n","c","g","h","a","j","y","k"]; Arr.sort (function(a,b){console.log(a.charcodeat (0), b.charcodeat (0)); // str.charcodeat (0) => Convert the 0th item of STR string to Unicode return a.charcodeat (0) -b.charcodeat (0); // a-z // return b.charCodeAt(0)-a.charCodeAt(0); // z-a }) console.log(arr);Copy the code

14, some ()

Checks if there are elements in the array that meet the criteria, returning true if there are, false if there are none

If there is one element that meets the condition, it will return true

Var arr =,4,6,2,7,9,0 [1]; Var bool=arr.some(function(item,index,arr){return item>5; }); console.log(bool);Copy the code

15, every ()

var bool=arr.every(function(item,index,arr){ }); Check whether all elements in the array meet the criteria. If one element does not meet the criteria, jump directly. If all elements meet the criteria, return true

Var arr =,4,6,2,7,9,0 [1]; Var bool=arr. Every (function(item,index,arr){return item>2; }); console.log(bool);Copy the code

16, the filter ()

The requirement is to return anything in the array greater than 5 to a new array

The first thing that comes to mind is a map, which only makes the original array and the new array the same length

        var arr1=arr.filter(function(item,index,arr){
           return item>5;
        });
        console.log(arr1);
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17, the reduce ()

The first bit of the array is traversed, the 0th bit is not traversed, and the subscript starts at 1

So value is the 0th entry in the array, and then every value is undefined

If you use return in a function, you assign the return value to a value in the loop

Var arr =,3,4,7,3,5,8,9 [10]; Arr. Reduce (function(value,item,index,arr){// The number of loops is the number of arrays. The 0th pass (starting with the 0th entry of the array) console.log(value); return value+1 })Copy the code

Cumulative summation of array elements

Reduce returns a value that is iterated until the last return

Var arr =,3,4,7,3,5,8,9 [10]; var sum=arr.reduce(function(value,item){ return value+item; }); console.log(sum);Copy the code

You want to sum the elements of a cumulative additive array, given the cardinality 100, which is the initial value

Var arr =,3,4,7,3,5,8,9 [10]; Var sum=arr.reduce(function(value,item){console.log(value); return value+item; }, 100);Copy the code

18, Array. IsArray ()

To check if it’s an array

Var arr = [1, 2, 3, 4]; var obj={a:1}; console.log(typeof arr); ES6 console.log(array.isarray (arr)); ES6 console.log(array.isarray (arr)); console.log(Array.isArray(obj));Copy the code

2. Math method

1. PI and the square root of 2

Math.PI; / / Π Math. SQRT2; // The square root of 2 constant can only be used and cannot be modifiedCopy the code

2, rounded up

1) Integer math.floor ()

A = Math. Floor (25.6); console.log(a);Copy the code

2) Round up math.ceil ()

B = math.h ceil (25.6); console.log(b);Copy the code

3, Round math.round ()

Error in method

C = Math. Round (25.5); console.log(c);Copy the code

The rounding of negative numbers is generally converted to a positive number

4. Maximum and minimum values

1) Minimum math.min ()

C = Math. Min,7,8,3,1,9,6,0,3,2 (4) the console. The log (c)Copy the code

2) Maximum math.max ()

C = Math. Max,7,8,3,1,9,6,0,3,2 (4) the console. The log (c)Copy the code

5, Random number math.random

c=Math.random()*10+1; // Random number between 1 and 10 console.log(c)Copy the code

Random numbers that are not integers are generally rounded

c=Math.random()*10+1; console.log(Math.floor(c)); // round downCopy the code

6. Other methods

String String method

1, the charAt ()

Gets the character of the subscript position, and STR [1]; The same

      var str="abcdef";
      console.log(str.charAt(3));
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2, charCodeAt ()

Converts characters at subscript positions to Unicode encoding

      var str="abcdef";
      console.log(str.charCodeAt(2));
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3, String. FromCharCode ()

Converts Unicode encoding to a string

n=String.fromCharCode(65); // Convert Unicode encoding to string console.log(n);Copy the code

4, STR. Concat ()

Concatenation string, effect equivalent to concatenation +

var str="abc"; var str1="def"; var str2=str.concat(str1); // var str2=str+str1; This is equivalent to concat console.log(str2);Copy the code

5、 indexOf() 、lastIndexOf()

The indexOf the search character is the same as the indexOf the array

Var arr = [{id: 1001, the name: "computer", price: 4999}, {id: 1002, the name: "motor", price: 1999}, {id: 1003, the name: "notepad", price: 9}, {id: 1004, the name: "textbook", price: 99}, {id: 1005, the name: "calculator", price: 149},]; Var arr1=arr.filter(function(item){return item.name.indexof (" this ")>-1}); console.log(arr1);Copy the code

6, the replace ()

Replace, like splice() in arrays;

Replace does not modify the content of the original character, and returns a new modified string

If two identical elements appear, only the element found the first time is modified

      var str="abcdecf";
      var str1=str.replace("c","z");
      str.replace()
      console.log(str,str1);
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7, slice (start, end)

Slice (starting at and ending before the subscript) intercepting copy strings are allowed to have negative values, which indicate backwards to forwards

var str="abcdecf"; Var s = STR. Slice (1, 2); console.log(s);Copy the code

8, the substring ()

var str="abcdecf"; Var s = STR. The substring (2, 4); Var s=str.substring(4,2); var s=str.substring(4,2); console.log(s);Copy the code

9, substr (start with subscript, truncate length);

var str="abcdecf"; Var s = STR. Substr (- 2, 5); // Start with the subscript number and intercept the length console.log(s);Copy the code

Split converts a string into an array using a separator

      var str="a,b,c,d,e,f";
      var arr=str.split(",");
      console.log(arr);
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11, reverse (); Array elements in reverse order or reverse order, this is an array method

var str="abcdefg"; var str1=str.split("").reverse().join(""); / / linking the console. The log (str1)Copy the code

12, string conversion case

1) change str.toLowerCase toLowerCase

console.log("ABC".toLowerCase()); // Convert the string to lowercaseCopy the code

2) STR. ToUpperCase is uppercase

console.log("abc".toUpperCase()); // Convert the string to uppercaseCopy the code

Date object

Create a date object

var date=new Date(); // Create a date objectCopy the code

2. Get the date

var date=new Date(); date.getFullYear(); // Get the year date.getYear(); Date.getmonth ()+1; date.getMonth()+1; // Get month from 0 date.getDate(); // Get the date date.getday (); // Get week from 0 0 is Sunday 1 is Sunday 1 date.gethours (); // Get the hour date.getminutes (); Date.getseconds (); // get date.getmilliseconds (); // Get milliseconds console.log(date);Copy the code

2. All the above dates can be set

var date=new Date(); date.setFullYear(2021); date.setMonth(12); // In January, the year +1 is rounded from 0; date.setDate(40); SetHours (date.gethours () + 2); // Set it to 2 hours after the present date.getutcfullyear (); // Any time with UTC is GMT date.tostring (); // Convert directly to the string date.tolocaleString (); // Convert to local (Windows) format time string date.toutcString (); // Convert to the Greenwich time string console.log(date);Copy the code

3. Timestamp

date.getTime(); // The number of milliseconds from 00:00:00 on January 1, 1970 to the presentCopy the code

As time goes by, this number will keep getting bigger and bigger, getting different values every millisecond, so this number will never be repeated

The browser thinks that if the address is the same each time, it checks the browser cache first and uses the local cache if it does

You can timestamp each visit to a different address to avoid local caching by the browser