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 stringarr.push(6); // Insert a new element at the end of the arrayArr. Push (June); // Insert multiple new elements at the end of the arrayA = arr. Push (June); // Returns the new length of the array by adding it to the tail element console.log(arr); console.log(a); Copy the codeReturns 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); // Inserts a new element at the head of the arrayArr. Unshift (3, 2, 1, 0). // Insert multiple new elements in the header of the arrayVar a = arr. Unshift (3, 2, 1, 0). // Returns the new length of the array console.log(arr);
console.log(a); Copy the codeReturns 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(); //pop has no arguments. Remove the last element of the arrayvar a=arr.pop(); //pop removes the last element of the array and returns the deleted element console.log(arr);
console.log(a);
Copy the codeReturn the deleted element
Shift () removes the first element of the array and returns the deleted element
Arr = [6]arr.shift(); // Delete the first element of the arrayvar a=arr.shift(); // Remove the first element of the array, and return the deleted element console.log(arr);
console.log(a);
Copy the codeReturns 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); // Array merge, return a new array, the original two arrays do not change console.log(arr2);
var arr3=arr.concat(0,-1,-2); // An array can merge elements as well as arrays. Multiple elements are merged with the original array console.log(arr3) var arr4=arr.concat(0,arr1,["A"."B"]); // concat can merge both elements and arrays console.log(arr4) var arr5=arr.concat(); // If you use concat directly, you can copy arrays 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]; // join returns each element of an array as a new string with the specified concatenation of charactersvar str=arr.join( ); // Merge arrays into strings, using commas by default, concatenated 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, it concatenates the elements of an array into a 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 without any arguments console.log(arr1);
The first argument is 0, which indicates the number of bytes from the 0th bit, and the second argument is not filled in, which means to the endvar arr1=arr.splice(0); // Move all data to the new array console.log(arr1);
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2) Splice can be used from front to back or from back to front
Var arr1 = arr. Splice (0, 3); // Remove 3 elements from bit 0 and return to arr1var arr1=arr.splice(-2); // The number of digits can be negative, and the number of digits can be negative, and the number of digits to delete is backwardVar arr1 = arr. Splice (0, 1, 1); // Starting at bit 0, delete 1 element, and insert element -1 at that position, replaceVar arr1 = arr. Splice (1, 0). // The last substitution bit of the array is 0Var arr1 = arr. Splice,2,10,11 (2); // replace the first two elements of the array with 10,11Copy the code3) Splice can insert one or more elements
Var arr = [1, 2, 3, 4, 5];Arr. Splice (2, 0, 1); // Inserts an element -1 in the second part of the array console.log(arr);
Arr. Splice (2, 0, 1, 2, 3, 4); // Insert multiple elements in the second part of the array console.log(arr);
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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); // start with the first bit and end with the fourth bitvar arr1=arr.slice(); // Copy the array arrvar arr1=arr.slice(0); // Copy the arrayvar arr1=arr.slice(3); // copy from the third bit to the tailvar arr1=arr.slice(-2); // Start from the last second to the endvar arr1=arr.slice(-3,-1); // From third to lastCopy the code9, 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}); // This is a new object with a different address console.log(index);
Copy the codeIf 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); // Select * from 1; console.log(index);
Var index = arr. IndexOf (3, 2); // Only the second 3 can be queried (subscript 4) console.log(index);
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Query all the same elements using a circular query
Var arr =,3,1,2,3,5,2,3,4,6 [1];var index=0; // Use a loop to find all elements in the array with a value of 3 and print the subscript while(true) { index=arr.indexOf(3,index);
console.log(index);
if(index===-1) break; If the value is -1, no further query is performed 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"); // Get the HTML element whose tag name is div
Divs. Pop (); // Error, cannot use array method directly instead of array
var arr=Array.from(divs); Divs = divs; divs = divs/ / ES5 writing = > var arr = Array. The prototype. Slice. Call (divs);Copy the code2) Add click events to all div elements
var divs=document.getElementsByTagName("div");
var arr=Array.from(divs); // Convert all divs into arrays
for(var i=0; i<arr.length; I++){// iterate over the number group to add click events to each div arr[i].onclick=clickHandler;
} function clickHandler() { console.log(this); var index=arr.indexOf(this); console.log(index); // To determine which element in the list is clicked } Copy the code11, 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 codeTraversal 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);
})
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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";
returnitem+10; // Use in mapreturnAdd the corresponding data to the corresponding subscript }); console.log(arr2); Copy the code
// Create a new array for elements greater than 4 (undefined)Var arr =,3,5,7,2,4,6,8 [1]; 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){
returna-b; // From small to big // 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){// Sort the charactersconsole.log(a.charCodeAt(0),b.charCodeAt(0)); // str.charcodeat (0) => Converts the 0th entry of the STR string to Unicode encoding 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){// Go through the list to see if there is an element greater than 5 return item>5;
});
console.log(bool);
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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){// Check whether all elements in the array are greater than 2 return item>2;
});
console.log(bool);
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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 -1// value is the last iterationreturn, the 0th pass (starts as 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);
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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){
// The argument after the function is the initial value of the sum 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);
// Check if the element is an arraytrueOtherwise returnsfalseThe 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; / / ΠMath.SQRT2; // The square root of 2 constant can only be used and cannot be modifiedCopy the code2, rounded up
1) Integer math.floor ()
A = Math. Floor (25.6); console.log(a);
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2) Round up math.ceil ()
B = math.h ceil (25.6); console.log(b);
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3, Round math.round ()
Error in method
C = Math. Round (25.5); console.log(c);
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4. Maximum and minimum values
1) Minimum math.min ()
C = Math. Min (4,7,8,3,1,9,6,0,3,2) console.log(c)
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2) Maximum math.max ()
C = Math. Max (4,7,8,3,1,9,6,0,3,2) console.log(c)
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5, Random number math.random
c=Math.random()*10+1; // A random number between 1 and 10 console.log(c)
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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 the Unicode encoding to a string console.log(n);
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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 the same as concat console.log(str2);
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5、 indexOf() 、lastIndexOf()
The indexOf the search character is the same as the indexOf the array
var arr=[
{id:1001,name:"Computer",price:4999},
{id:1002,name:"Motor",price:1999},
{id:1003,name:"Notepad",price:9},
{id:1004,name:"Book",price:99},
{id:1005,name:"Calculator",price:149}, ]; // fuzzy search 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);
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8, the substring ()
var str="abcdecf";
Var s = STR. The substring (2, 4); // Similar to slice// Substring does not allow negative numbers. All negative numbers are before 0, so negative numbers are 0// You can intercept assignments backwardsVar s = STR. The 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); // Cut the length from the index console.log(s);
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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(""); / / : console.log(str1)
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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 code2. Get the date
var date=new Date();
date.getFullYear(); // Get the yeardate.getYear(); // Get the year from 1900, there is a problemdate.getMonth()+1; // Get months from 0date.getDate(); // Get the datedate.getDay(); // Get week from 0, 0 is Sunday, 1 is Sundaydate.getHours(); // Get the hourdate.getMinutes(); // Get minutesdate.getSeconds(); / / for secondsThe date. The 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); // Set the carrydate.setHours(date.getHours() + 2); // Set it to 2 hours after the present timedate.getUTCFullYear(); // All time with UTC is Greenwich mean timedate.toString(); // Convert directly to a stringdate.toLocaleString(); // Convert to a local (Windows) format time stringdate.toUTCString(); // Converts to a Greenwich mean 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 codeAs 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
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