JSFiddle - React, Tailwind, and code Playground

by Vladymyr Shevchuk

JavaScript

const sum = 5;
const items = [1,2,5,3,4,8]; 

function findByIndex(arr, base){
  	let listIndex = [];
  
    for (let i=1, total = arr.length; i < total; i++) {
      let item = base - i;
      let itemIndex = arr.indexOf(item);
      let restIndex = arr.indexOf(i);

      if (itemIndex > 0 && restIndex >= 0) {

      listIndex.push(itemIndex);
      listIndex.push(restIndex);

      return listIndex;
    }
  }
}

findByIndex(items, sum); // [4, 0]