Summary of usage of reduce in JavaScript (with code)

Recently often in the project to see others using reduce processing data, is a cow break, very dreamy, rather than their own pondering.

First look at the grammar

Array.reduce (function (total, CurrentValue, Currentindex, arr), initialvalue);/*  total: Required. The initial value, or the return value after the calculation has ended.  CurrentValue: Required. The current element.  currentindex: Optional. The index of the current element;                       arr: Optional. The array object to which the current element belongs.  initialvalue: Optional. The initial value passed to the function, equivalent to the initial value of total. */

Common usage

Array summation

Const ARR = [23];const sum = arr.reduce (total, num)-+ total + num);<!--Set initial value summation-->const arr = [12, 34, 23];const sum = arr.reduce (total, num)-= total + num,  Sum <!--An array of initial values with 10-->var result = [  {subject: ' Math ', score:88},  {subject: ' Chinese ', score:95},  {subject: ' 中文版 ', score:80}];const sum = Result.reduce ((prev, cur) = prev + cur.score, 0); Const SUM = result.reduce ((prev, cur) = prev + Cur.score, -10);  Total deduction 10 points

Array Maximum Value

Const A = [23,123,342,12];const max = A.reduce (function (Pre,cur,inde,arr) {return pre>cur?pre:cur;}); 342

Advanced usage

Usage in array objects

<!--such as "Boss, Dick and old three"-->const Objarr = [{name: ' Boss '}, {name: ' Dick '}, {name: ' Old three '}];const res = objarr.reduce (PRE, cur , index, arr) = = {  if (index = = 0) {    return cur.name;  }  else if (index = = = (arr.length-1)) {    return pre + ' and ' + Cur.name;  }  else {    return pre + ', ' + Cur.name;  }}, ');

To find the number of occurrences of a letter in a string

Const STR = ' Sfhjasfjgfasjuwqrqadqeiqsajsdaiwqdaklldflas-cmxzmnha '; const RES = Str.split ("). Reduce ((prev, cur) + = { Prev[cur]? prev[cur]++: prev[cur] = 1; return prev;}, {});

Array to array

<!--according to certain rules to the group-->var arr1 = [2, 3, 4, 5, 6]; The square var of each value Newarr = Arr1.reduce ((prev, cur) + = {Prev.push (cur * cur); return prev;}, []);

Array to Object

<!--remove stream by id-->var streams = [{name: ' Technology ', id:1}, {name: ' design ', id:2}];var obj = Streams.reduce ((prev, cur) = = {Prev[cur.id] = cur; return prev;}, {});

Advanced usage

Multi-dimensional overlay execution action

<!--the results of different subjects are different,-->var result = [{subject: ' Math ', score:88}, {subject: ' Chinese ', score:95}, {Subje CT: ' 中文版 ', score:80}];var dis = {math:0.5, chinese:0.3, English:0.2};var res = result.reduce (prev, cur ) = Dis[cur.subject] * cur.score + prev, 0);<!--to increase the difficulty, the goods corresponding to different countries exchange rates, the total price-->var prices = [{price:23}, {Price:4 5}, {Price:56}];var rates = {us: ' 6.5 ', EU: ' 7.5 ',};var initialstate = {ustotal:0, eutotal:0};var res = Prices.reduce  (Prev1, Cur1) and Object.keys (rates). Reduce ((Prev2, cur2) = {Console.log (prev1, Cur1, Prev2, CUR2);  prev1[' ${cur2}total '] + = Cur1.price * RATES[CUR2]; return PREV1;}, {}), initialstate), var managereducers = function () {return function (state, item) {return Object.keys        (rates). Reduce ((nextstate, key) = {state[' ${key}total '] + = Item.price * Rates[key];      return state;  }, {}); }};var res1= Prices.reduce (Managereducers (), initialstate);

Flat a multidimensional array

var arr = [[1, 2, 8], [3, 4, 9], [5, 6, 10]];var res = Arr.reduce ((x, y) + x.concat (y), []);

Object array de-weight

Const HASH = {};  Chatlists = Chatlists.reduce ((obj, next:object) and {    const Hashid = ' ${next.topic}_${next.stream_id} ';    if (!hash[hashid]) {      hash[' ${next.topic}_${next.stream_id} '] = true;      Obj.push (next);    }    return obj;  }, []);