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This article introduces the knowledge of "how to use reduce () in JavaScript". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

1. Definition and usage

The reduce () method takes a function as an accumulator, and each value in the array is reduced from left to right, and is finally evaluated as a value.

Note: reduce () does not execute callback functions for empty arrays.

2. Grammar

Arr.reduce (function (prev, cur, curIndex, arr) {

...

}, init)

3. Parameters

Prev: required, the initial value init, or the return value after each calculation

Cur: required, current element

Index of curIndex:cur

Arr: the array arr to which the current element belongs

Init: the initial value passed to the function.

4. Examples

Var arr = [5, 4, 8, 6, 6, 8, 2, 6, 22, 8, 6]

one

Summation

Sum = arr.reduce (function (prev,cur) {

Console.log (prev, cur)

Return prev + cur

}, 0)

/ / the print result is as follows:

0 5

5 4

9 8

17 6

23 8

31 2

33 6

39 22

61 8

69 6

seventy-five

Find the maximum value of the array item

Max = arr.reduce (function (prev, cur) {

Console.log (prev, cur)

Return Math.max (prev, cur)

}, 0)

/ / the print result is as follows:

0 5

5 4

5 8

8 6

8 8

8 2

8 6

8 22

22 8

22 6

twenty-two

Array deduplication

NewArr = arr.reduce ((prev, cur) = > {

Console.log (prev, cur)

Prev.indexOf (cur) = =-1 & & prev.push (cur)

Return prev

}, [])

/ / the print result is as follows:

[] 5

[5] 4

[5, 4] 8

[5, 4, 8] 6

[5, 4, 8, 6] 8

[5, 4, 8, 6] 2

[5, 4, 8, 6, 2] 6

[5, 4, 8, 6, 2] 22

[5, 4, 8, 6, 2, 22] 8

[5, 4, 8, 6, 2, 22] 6

[5, 4, 8, 6, 2, 22]

Calculate the number of occurrence of string letters

StrNum = str.split ('') .reduce ((strObj, cur) = > {

Console.log (strObj, cur)

StrObj [cur]? StrObj [cur] + +: strObj [cur] = 1

Return strObj

}, {})

/ / the print result is as follows

{}'h'

{h: 1}'e'

{h: 1, e: 1}'l'

{h: 1, e: 1, l: 1}'l'

{h: 1, e: 1, l: 2}'o'

{h: 1, e: 1, l: 2, o: 1}

Convert a two-dimensional array to an one-dimensional array

Var arr1 = [[1jue 2], [1JI 4je 3], [7je 8rem 9]]

Arr2 = arr1.reduce ((xQuery y) = > x.concat (y), [])

/ / [1, 2, 1, 4, 3, 7, 8, 9]

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