Reducing Values by Reduce Method

The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.

Flatten Array of Objects

The example below shows how to flatten an array of objects into a single object.

1let array=[{
2    key:'Moiz',
3    value:1    
4},
5{
6    key:"Hello",
7    value:2  
8},
9{
10    key:"Hassan",
11    value:9  
12},
13]
14let result=array.reduce((obj,present)=>({...obj,[present.key]:
15    present.value}),{})
16console.log("result",result)
17

Map Using Reduce

As another example of using the initial value parameter, consider the task of calling a function on an array of items, returning the results in a new array. Since arrays are ordinary values and list concatenation is an ordinary function, we can use reduce to accumulate a list, as the following example demonstrates:

1let array=[{
2    key:'Moiz',
3    value:1   
4},
5{
6    key:"Hello",
7    value:2   
8},
9{
10    key:"Hassan",
11    value:9  
12},
13]
14function map(array, fn) {
15 return array.reduce(function(newList, item) {
16 return newList.concat(fn(item));
17 }, []);
18}
19let a=map(array,function(n){return n.value * n.value})
20console.log(a)
21  

Finding Unique Values

Here is an example that uses reduce to return the unique numbers to an array. An empty array is passed as the second argument and is referenced by prev

1var arr = [1, 2, 1, 5, 9, 5];
2let arrSort=arr.reduce((prev, number) => {
3 if(prev.indexOf(number) === -1) {
4 prev.push(number);
5 }
6 return prev;
7}, []);
8console.log("arrSort",arrSort)
9  

Array Sum

This method can be used to condense all values of an array into a single value:

1
2  var arr = [1, 2, 1, 5, 9, 5];
3let sumArray=arr.reduce(function(a,b){
4    return a+b
5})
6console.log("sumAraay",sumArray)
7let sumArry=[10].reduce(function(a,b){
8   return a+b 
9},1)
10console.log("sumArray2",sumArry)
11