Array

(1), definition of array

Array is an ordered collection of values

JavaScript array is untyped; array elements can be of any type, And different elements of the same array may also have different types.

Each value is called an element, and each element has a position in the array.

(2). Create an array.

1. Use array literals to create an array. The simplest way is to separate the array elements with commas in square brackets.

　eg:var empty = []; //Array with no elements

　eg:var primes = [2,3,4,5,7]; //Array with 5 values Array

　eg:var misc = [1.1,true,"a",]; //There are 3 elements of different types and a comma at the end. The value in the array literal does not have to be a constant, it can be anything expression.

eg:var base = 1602;

var table = [base+1,base+2,base+3];

If a certain element in the array literal is omitted value, omitted elements will be assigned the value undefined.

var count = [1,,3]; //The array has 3 elements, and the middle element has a value of undefined

var undefs = [,,]; //The array has two elements elements, all are undefined. The syntax of array literals allows an optional trailing comma, so [,,] has only two elements instead of three.

2. Calling the constructor Array() is another way to create an array. The constructor can be called in three ways.

　①There are no parameters when calling: var a = new Array(); This method creates an empty array without any elements, which is equivalent to the array literal [].

　②There is a numeric parameter when calling, which specifies the length: var a = new Array(10) When the number of required elements is known in advance, this form of Array() constructor can be used to pre-allocate An array space.

　③ Display and specify two or more array elements or a non-numeric element of the array: var a = new Array(5,4,3,2,1,”test”); In this form, The parameters of the constructor will become the elements of the new array. Using array literals is much simpler than using the Array() constructor like this.

(3), Reading and writing of array elements

Use the [] operator to access an element in the array element. Array references are on the left side of the square brackets. Enclosed in square brackets is an arbitrary expression that returns a nonnegative integer value. Use this syntax to both read and write an element of an array.

1. You can access a specific element by specifying the array name and index number. Eg:mycars[0]

2. If you need to modify the value in the existing array, just add a new value to the specified index: Eg:mycars[0]="Opel";

(4), Array length

Definition: The length attribute can set or return the number of elements in the array. (Starting from 1)

The length property of an array is always greater than the subscript of the last element defined in the array

1. For those regular elements that have consecutive elements and start with element 0 For arrays, the length attribute declares the number of elements in the array.

　 The length property of the array is initialized when the array is created using the constructor Array(). When a new element is added to the array, the value of length is updated if necessary.

Setting the length property can change the size of the array. If set to a value smaller than its current value, the array will be truncated and its trailing elements will be lost. If the set value is greater than its current value, the array will grow and new elements will be added to the end of the array, with their value being undefined.

eg:[].length ==0

[1,2,3].length ==3

(5). Check whether it is an array

1. Use the instanceof operator to determine whether an object is an array

Instanceof is a binary operator. The left operand is an object. If not, it returns false. The right operand is a function object or Function constructor, returns false if not. The principle is to determine whether the prototype chain of the object of the left operand has the prototype attribute of the constructor of the right operand.

eg: arr instanceof Array

2. You can use Array.isArray(arr)

This is a new Array method in ES5, which is a static method of the Array object Function used to determine whether an object is an array.

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