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September 20, 2021 05:03 pm GMT

Learn Javascript Reduce method with 5 examples

The reduce method applies a function to every item of an array and accumulates the result iteratively from a starting point then returns a single value/object.

Some examples of the use of the reduce method

Sum the items of an array:

[3, 5, 4, 3, 6, 2, 3, 4].reduce((a, i) => a + i, 0 );// For clarity the above code is the same as [3, 5, 4, 3, 6, 2, 3, 4].reduce(function(a, i){return (a + i)}, 0 );

Find the maximum in an array:

Here in each iteration we return the max between the accumulator and the current item and in the end we have the max of the entire array.

[3, 5, 4, 3, 6, 2, 3, 4].reduce((a, i) => Math.max(a, i), -Infinity );

Removing duplicates in an array:

We check if the current value has index on the accumulator array if not is going to return -1 hence is not in the array and we can add it.

let dupes = [1,2,3,'a','a','f',3,4,2,'d','d']let withOutDupes = dupes.reduce((noDupes, curVal) => {  if (noDupes.indexOf(curVal) === -1) { noDupes.push(curVal) }  return noDupes}, [])

Extract properties in an array of objects returning an array

let obj = [  {name: 'Alice', job: 'Data Analyst', country: 'AU'},  {name: 'Bob', job: 'Pilot', country: 'US'},  {name: 'Karen', job: 'Software Eng', country: 'CA'},  {name: 'Jeremy', job: 'Artist', country: 'SP'},]let ppl = obj.reduce((persons, curPerson) => {  persons.push([curPerson.name, curPerson.job, curPerson.country])  return persons}, [])// You can also do this with maplet pplm = obj.map((curPerson)=>[curPerson.name, curPerson.job, curPerson.country])

Flattened an array of arrays

This is only 1 level deep but it you can adapt this with a recursive function, but i'm not that fan of making recursive things on javascript 

let flattened = [[3, 4, 5], [2, 5, 3], [4, 5, 6]].reduce(  (singleArr, nextArray) => singleArr.concat(nextArray), [])// results is [3, 4, 5, 2, 5, 3, 4, 5, 6]

Original Link: https://dev.to/ramgendeploy/learn-javascript-reduce-method-with-5-examples-128n

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