JSFiddle - React, Tailwind, and code Playground

JavaScript

var x = 237;
var array = [100, 200, 300, 400];
var copy_of_array = array.slice();
var my_sort_Function = function (a, b) { 
	return Math.abs(x - a) - Math.abs(x - b)
};
var index = array.indexOf(copy_of_array.sort(my_sort_Function)[0]);

document.body.innerHTML = "Index: " + index + " Value: " + array[index];