13. Array objects

13.1 The difference between C and JS arrays

C language array:

Elements have the same data type
Continuous memory storage
Access elements by numeric subscript
Memory:

Js arrays: Arrays that are completely simulated by objects

Element data types can be different
It doesn’t have to be contiguous, but if the value is an object, there’s an address in memory
You can’t subscript numbers, you can subscript strings
Memory:

13.2 Creating an Array

let arr = [1.2.3]
let arr = new Array(1.2.3) / / [1, 2, 3]
var arr = new Array(3) // [empty × 3
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How do I split a string to generate an array?

var arr = '1, 2, 3'.split(', ') / / / "1", "2", "3"]

var arr = '123'.split(' ')    / / / "1", "2", "3"]


/ / 2. Use Array. The from
Array.from('123')  / / / "1", "2", "3"]

Array.from(123)   / / []

Array.from(true)  / / []

Array.from({name:'frame'}) / / []

Array.from({0:'a'.1:'b'}) / / []

Array.from({0:'a'.1:'b'.length:2}) //["a", "b"] the resulting array is a pseudo-array

Array.from({0:'a'.1:'b'.length:1}) //["a"]
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What is a pseudo-array?

There is no prototype of an array in the pseudo-array, which is the POP () Push () API

Pseudo-array generation method:

Object add length, forcibly generated:Array. The from ({0: 'a', 1: 'b', length: 2}) / / / "a", "b" that generated arrays is false
var divList = document.querySelectorAll('div')You get a pseudo-array

How do I join two arrays

Arr. Concat (arr2) returns a new array, unchanged

The use of the slice

Section:

var arr = [1.2.3.4.5.6.7.8.9]

// Enter the starting coordinates
arr.slice(2)  //[3, 4, 5, 6, 7, 8, 9]

arr  //[1, 2, 3, 4, 5, 6, 7, 8, 9]

// Enter 0 for shallow copy
arr.slice(0)  //[1, 2, 3, 4, 5, 6, 7, 8, 9]
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13.3 Deleting an Element from an Array

Mentally retarded delete: Deletes elements but leaves them empty

var arr = ['a'.'b'.'c']
delete arr[0]  //true
arr   //[empty, "b", "c"] length does not change, delete is a mental retarded
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Such arrays are called sparse arrays and are only bug-generating

Don’t change length arbitrarily

Correct deletion of posture:

Delete the first element:.shift()

Delete the last one: .pop( )

Replace the middle element:.splice(begin, length, item)

###13.4 Array traversal

Retarded in: prints elements other than length

It’s good to iterate with for

ForEach traverses the array

For is the same as forEach except for a break continue

For is the keyword, block-level scope, and forEach is the function scope

Iterate over the number of errors encountered

Cross-border undefined

###13.5 Find elements

indexOf(): Finds elements by index
.find((x)=> (x%5) === 0)) : Find the element according to the condition, : the element that x divisible 5,The element is returned
.findIndex((x)=> (x%5) === 0)) : Find the element according to the condition, : the element that x divisible 5,The index is returned