JSFiddle - React, Tailwind, and code Playground

JavaScript

var query = "SELECT a, b, c, d FROM";
var iSelect = query.indexOf("SELECT");
var selLen = "SELECT".length;
var iFrom = query.indexOf("FROM");
if (iSelect >= 0 && iFrom >= 0) {
    query = query.replace(query.substring(iSelect + selLen, iFrom), " COUNT(*) ");
    console.log(query);
}