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An array of

Know the array

Arrays are our old friends. In general, arrays are used to store values of the same data type, for example, an array has a list of objects in memory, a list of strings, a list of values, etc. In JS, arrays can store different types of values, but this is not recommended

In addition, the physical addresses of the array are adjacent, that is, the addresses stored in the computer.

Create an array

New is often used when JS creates arrays. We can also assign values to arrays at creation time

var ary = new Array('1'.'2')
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And of course the most common and recommended creation method

var ary = ['1'.'2']
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Array traversal

If you want to find an element in an array or if you want to know the length of an array, the most common thing we do is walk through an array

Probably the most common traversal we do is to use a for loop to traverse an array

for (var i = 0; i < ary.length; i++) {
    alert(ary[i])
}
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However, some iteration methods of arrays have been encapsulated in JS to make array traversal faster

  1. ForEach: Cannot be terminated in the middle of a procedure; you need to traverse each element to find it.

    const arr = ['我'.'you'.'he']
   arr.forEach((item,index) = > {
       if(item === 'you') {
           console.log(index)
       }
   })
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  2. Some: The some loop can be terminated by retrun true after the corresponding item is found

     const arr = ['我'.'you'.'he']
   
   arr.some((item,index) = > {
       if(item === "You") {
           return true;
           // Terminate the loop}})Copy the code

  3. Every: Returns the Boolean value of whether each item satisfies the judgment condition

const arr = [
       {id:1.name:'clothes'.state:true},
       {id:2.name:'fruit'.state:true},
       {id:3.name:'computer'.state:true}]const result = arr.every(item= > item.state == true)
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Array capacity

Normally speaking, the capacity of the array in common languages can not be automatically changed, we need to expand the operation, expanding the array in JS is relatively simple

ary.length = 10
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Or we can just add elements

ary[ary.length] = 9
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Or use the push method to add elements to the end of the array

numbers.push(11)
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But if we’re comparing low-level arrays we need to do some complicated things

If we want to increase the array is the need to speak to copy an array within an array of length is longer, the linked list and array is so so, the difference between a linked list can be infinite expansion, but an array, the array of expansion is a time-consuming, enter oneself for an examination, delete and add, because and expansion is essentially the same, but relative to the chain table lookup or fast, You can do this by using subscripts directly.

Increase for deletion

  1. Adding an element: Adding an element is adding an element. We’ve already mentioned expansion, but what if we want to add an element to the middle of an array?

    var ary =['1'.'2'.'3'.'4']
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    Here we want to add an element between 2 and 3, what do we do?

    for (var i = ary.length+1; i = 3; i--) {
    ary[i] = ary[i-1]
}
ary[3] = '5'
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    The idea is that we can use a for loop to move the position of the element after 2 backwards, starting at the end, stopping at 3 and assigning it directly to the array subscript to add and insert at any position

  2. Remove elements

    Pop () is used to remove the element at the end of the array

    var ary =['1'.'2'.'3'.'4']
ary.pop()
console.log(ary)
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    But if we want to delete an element anywhere in the array the same way we would add an element anywhere in the array

    var delet = 2
for (var i = delet; i < ary.length; i++) {
    ary[i] = ary[i+1]
}
ary.pop()
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    You can declare the subscript of the array you want to delete, then start at the target location, let the subscript element overwrite the previous element, and finally use pop() to delete the element at any location

  3. Changing an element anywhere in the array and getting an element anywhere in the array just use subscripts!

    ary[1] = '3'
console.log(ary[0])
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