JSFiddle - React, Tailwind, and code Playground

HTML

<div id="page_1"></div>

JavaScript

var runStartFunction;

if (document.getElementById("page_1") != null) {
    alert("Page 1");
    runStartFunction = function () {
        alert("what I want");
    };
}

if (document.getElementById("page_2") != null) {
    alert("Page 2");
    runStartFunction = function () { 
        alert("what I don't want");
    };
}

runStartFunction();