Null Variable in Javascript Code

Question

It keeps saying my variable is null even though it is assigned to a button in HTML. It doesn't allow the button press to work. The variable is "clickMe"

var yourName;   //global variable accessible to all functions

function showAnotherMessage() {
    alert("Hi " + yourName + ".\nThis is an alert message is no longer defined\nin the HTML but in a JavaScript file");
}

function init() {
    yourName = prompt("Hi. Enter your name.\nWhen the browser window is first loaded\nthe function containing this prompt window is called.", "Your name");
    var clickMe = document.getElementById("buttonclick");
  clickMe.onclick = showAnotherMessage;
    }

window.onload = init();

var clickMe = document.getElementById("buttonclick"); ... do you have a html element with id="buttonclick" - exactly "buttonclick", not "buttonClick" or "buttonclick " — Jaromanda X
– Jaromanda X, Commented Apr 5, 2017 at 23:57
yes, the id is assigned to a button. <p><button type="button" id="buttonclick">Click Me!</button></p> — Benjamin Inverno
– Benjamin Inverno, Commented Apr 5, 2017 at 23:58

Dai · Accepted Answer · 2017-04-06 00:04:42Z

2

Avoid window.onload because it's fired too late in the page lifecycle - it's only fired after everything is loaded, including all images - which can be up to several seconds after the page has loaded.

Instead, use the DOM events API, and use the DOMContentLoaded event:

window.addEventListener('DOMContentLoaded', function(e) {

    // page startup code goes here
} );

Another reason to avoid window.onload = ... is that it will overwrite any previous event-handler for the onload event, whereas addEventListener does not remove any previous event handler.

answered Apr 6, 2017 at 0:04

Dai

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