Arrays for Getting Started with JavaScript [Must-read for Beginners]

1. Define the array.

　　There are two ways to define an array:

　　1. var arr1 = []; //Define an empty array

　　 2. var arr2 = [1,2,3,"str1","str2"]; // Define an array with 5 elements.

　 3. var arr3 = new Array(3); //Define an empty array

　 4. var arr4 = new Array(1,2,3,"str1","str2"); //Define an empty array with a specified length is an array of 5.

2. Reading and writing of array elements.

　　 arr[0]; //Read the first array element

　 arr[0] = "str1"; //Change the value of the first element of the array.

3. Sparse array.

　　A sparse array represents an array with non-consecutive indexes starting from 0. Usually the length of the array represents the number of elements in the element. If the array is sparse, the length attribute value will be greater than the number of elements.

　 The in operator is used to detect whether an element exists at a certain position. Note that undefined is also considered to exist.

　　For example: var a1 = [,,];

　　var a2 = new Array(3);

　　 0 in a1; //true, because a[0] has undefined elements

　　0 in a2; //false , a2 has no element at index 0

IV. Array length

　　The length attribute is used to mark the length of the array

　　For example: var arr = [1,2,3,4,5];

　　　arr.length;　/ /5 The arr array has 5 elements

5. Adding and deleting array elements

　 push: //Add an element at the end of the array

var arr = [1,2,3];

　 arr.push( 4,5); //arr becomes [1,2,3,4,5]

　　delete: //Delete an element at a certain position in the array

　　var arr = [1,2,3]

　　delete arr [1]　 //arr becomes [1,,3]

　　1 in arr　　　 //false

6. Array traversal

　　Array traversal is usually implemented using the for statement

　　var arr = [1,2, 3,4,5];

　for(var i = 0.i

　if(!a[i]) continue;　　　　//Skip null, undefined and non-existent elements

　　　}

7. Multi-dimensional array

　　 A multi-dimensional array is an array of elements or an array

　　 For example: var arr = [[1,2,3],[,4,5,6]];

　　 arr[1 ][1]; // 5

8. Array methods

　 1. join() Used to convert all elements in the array into strings and connect them together. You can also customize the connection characters

　 var arr = [ 1,2,3];

　　 arr.join();　　 // => "1,2,3"

　　　 arr.join("=="); // => "1==2= =3";

　　 2. reverse() Used to reverse the order of array elements

　　 var arr = [1,2,3];

　　 arr.reverse(); // The arr array becomes [3,2 ,1]

　 3. sort(); // Used to sort the elements in the array. A function can be passed in for sorting. If it is empty, it will be sorted alphabetically. The undefined elements are sorted to the end

　　var arr = [1,2,3];

　　a.sort(function(a,b){

　　　return a-b; //Sort standard negative number 0 positive number, the comparison result returns the small one first The one

　　　}); //The value of the arr array is [1,2,3]　If the second condition becomes b-a, the result is [3,2,1]

　　 4. concat()　//Used for Combine a new array and return a new array

　　var arr = [1,2,3]

　　 arrnew = arr.concat(4,5)　//The arrnew array is [1,2,3,4,5 | / Used to return an array composed of elements in the specified range of the array. If a parameter is entered, it is the array from this parameter to the end. The two parameters are, the first parameter is the starting position, and the second parameter is the number.

　　　 var arr = [1,2,3,4,5];

　　 var arr1 = arr.slice(2); //[3,4,5]

　　 var arr2 = arr.slice(1,3 ); //[2,3]

　　 6. splice() 　 Delete or add elements. It will change the original array itself, which is equivalent to the reference (ref) in C#. The original array is an array composed of deleted elements, and the return value is an array composed of remaining elements.

　　　var arr = [1,2,3,4,5];

　　var arr1 = arr.splice(1,3); //arr is [2,3,4], and the returned array arr1 is [1 ,5]

　　var arr2 = [1,2,3,4,5];

　　var arr3 = arr2.splice(2,0,'a','b'); //Delete starting from the 2nd position , delete two elements, and then insert 'a', 'b' from that position; arr2 is [] because no elements are deleted, arr3[1,2,'a','b',3,4,5]

　　 7. push() and pop() Add or delete an element at the end of the array. When adding, it returns the last element added. When deleting, it returns the last element added. The return value is the deleted element.

　　　The push() function adds an element to the end of the array.

　　　pop()　 Function deletes the last element of the array.

　　　var arr = [1,2,3]

　　　 arr.push(4); pop(); //arr1 is [1,2]

　　 8. unshift() and shift()

　　 shift(), unshift() and push(), pop() are just operations on the head of the array rather than the tail.

　　　shift()　Removes an element from the head of the array, and the return value is the deleted element.

　　　unshift() Adds an element to the head of the array and returns the last element added.

　　　var arr = [1,2,3];

　　　var a = arr.shift(); //arr becomes [2,3] a is 1

　　var arr1 = [1,2,3];

　　 var b = arr1.unshift([4,5]); //arr1 becomes [4,51,2,3], b is 4 Return to the last one added, add 5 first and then add 4

　　9. toString() and toLocaleString() Convert the array into a string

　　 var arr = [1,2,3]

　　 arr.toString(); // Generate "1,2,3" and join without using any parameters ()it's the same.

2. Array methods in ECMAScript

　　1. forEach()　forEach() traverses the array from beginning to end and calls the specified function for each element.

var arr = [1, 2, 3, 4, 5];

var sum = 0;

arr.forEach(function (value) {

sum + value;

});
document.write(sum ); //sum ends up being 15

　　 2. map() The map() method passes each element of the called array to the specified function and returns an array.

var arr = [1, 2, 3, 4, 5];

var arr1 = arr.map(function (value) {

return value + 1;

});

document.write(arr1.join() ); //arr1 is [2,3,4,5,6]

　　 3. filter() Filter() filters. The returned elements are a subset of the calling array, filtering out elements that do not meet the conditions.

var arr = [1, 2, 3, 4, 5, 6];

var arr1 = arr.filter(function (i) { return i % 2 == 0 });

document.write(arr1.join ()); //arr1 is [2,4,6]

4. every() and some()

Every() returns true if and only if all elements in the array call the judgment function and return true true. Stop traversing when false is returned for the first time.

　　　some() returns true when there is an element in the array and the judgment function is called to return true. Stop traversing when true is returned for the first time.

var arr = [1, 2, 3, 4, 5, 6];

var a = arr.every(function (x) { return x > 3; }); var b = arr.some(function (y){ return y > 3; });

           document.write("The value of a is: " + a); // The value of a is false, not all elements in a are greater than 3

         document.write(" The value of b is: " + b); //The value of b is true, and there are elements in b greater than 3

　　 5. reduce() and reduceRight()

　　　reduce()　 Combine the elements in the array with the specified function , generates a single value, the first parameter is the simplified operation function, and the second parameter is the initial value passed to the function. The final result is the initial value and is calculated again based on the combined function and the final result. The second parameter, the initial value, can be omitted. When the initial value is omitted, calculation starts directly from the first element.

var arr = [1, 2, 3, 4, 5, 6];

var count = arr.reduce(function (x, y) { return x + y; },0);

document.write(count );

　　　reduceRight();　The only difference from reduce() is that it selects elements from right to left for calculation.

　　 6. indexOf() and lastInsexOf()

　　 indexOf()　 indexOf() returns the index of the first element found from the beginning to the end.

　　　lastIndexOf()　lastIndexOf() searches for elements in reverse order and returns the index of the first found element.

var arr = [1, 2, 3, 2, 1];
var i = arr.indexOf(2); //Search from beginning to end, the first time 2 is encountered is arr[1], so 1 is returned
var j = arr.lastIndexOf(2); //Search from the end to the beginning. The first time 2 is encountered is arr[3], so 3 is returned. document.write(i + "
");
document .write(j);

　7. Array.isArray(); //Determine whether an object is an array object

var arr = [1, 2, 3];

var str = "str1";
document.write( Array.isArray(arr)); //return true arr is an array object
        document.write(Array.isArray(str)); //return false str is a string, not an array object