JSFiddle - React, Tailwind, and code Playground

JavaScript

var x = 237;
var array = [100, 200, 300, 400];
var index = array.indexOf(array.sort( (a, b) => Math.abs(x - a) - Math.abs(x - b) )[0]);

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