JSFiddle - React, Tailwind, and code Playground

JavaScript

var list = [1, 4, 2, 4];
var sum = 8;
var outputList = [];

for (var i = 0; i < list.length; i++) {
	var check = sum - list[i];
	if (list.indexOf(check) !== -1 && list.indexOf(check) !== i) {
  	outputList.push(list[i], check);
    break;
  }
}

alert(outputList);