JavaScript's concat method instance code (array connection)

JavaScript concat() method

Definition and usage

The concat() method is used to concatenate two or more arrays.

This method does not change the existing array, but returns a new array.

Purpose: Merge 2 or more arrays

Variable name	value1,value2,value3,…,valueN
Detailed description	Value or array (object can be passed)
Is it necessary to pass	no

Note that concat returns a shallow copy

Example:

1. Connect 2 arrays

const arr = [1, 2, 3].concat([4, 5])
(arr) // [1,2,3,4,5]

2. Connect 3 arrays

const arr1 = [1, 2]
const arr2 = [3, 4]
const arr3 = [5, 6]
const arr4 = (arr2, arr3)
(arr4) // [1, 2, 3, 4, 5, 6]

3.Connect values to array

const arr1 = [1, 2]
const arr2 = 3
const arr3 = [5, 6]
const arr4 = (arr2, arr3)
(arr4) // [1, 2, 3, 5, 6]

Interesting knowledge

talk is cheap,show you my code

const arr1 = [[1]]
const arr2 = [3, 4]
const arr3 = [5, 6]
const arr4 = (arr2, arr3)
(arr4) // [[1], 3, 4, 5, 6]
arr1[0].push(2)
(arr4) // [[1, 2], 3, 4, 5, 6]

const arr1 = [1]
const arr2 = [3, 4]
const arr3 = [5, 6]
const arr4 = (arr2, arr3)
(arr4) // [1, 3, 4, 5, 6]
(2)
(arr4) // [1, 3, 4, 5, 6]

To put it simply, if there is a reference type in the connected array, then this reference type is shared. Actually, what I said at the beginning was a shallow copy, and you should understand it.

The object can be connected

const arr1 = [1]
const arr2 = [3, 4]
const arr3 = {
  a: 1,
  b: 2
}
const arr4 = (arr2, arr3)
(arr4) // [1, 3, 4, {a:1, b:2}]

The object has an attribute indicating whether to expand when using the concat method (note that it is expanded when used as a parameter)

Arrays are expanded by default
Objects are not expanded by default

Array expansion by default

const arr1 = [1]
const arr2 = [3, 4]
const arr3 = (arr2)
(arr3) // [1, 3, 4]  arr2Expanded

const arr1 = [1]
const arr2 = [3, 4]
arr2[] = false
const arr3 = (arr2)
(arr3) // [1,[3,4]] arr2Not expanded

Objects are not expanded by default

const arr1 = [1]
const obj2 = {
  a: 1,
  b: 2
}
const arr3 = (obj2)
(arr3) // [1,{a:1, b:2}] Objects are not expanded by default

const arr1 = [1]
const obj2 = {
// Note that you need to add the length attribute in this place, if you want to expand it  length: 2,
  0: 2,
  1: 3
}
obj2[] = true
const arr3 = (obj2)
(arr3) // [1,2,3]

Concat method of string

Example

const str1 = 'aa'
const str2 = ('bb')
(str2) // aabb

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