Getting “TypeError: Cannot set property 'onclick' of null”

This error indicates that when this code is executed, there is no element rendered in the browser that can be found with the id of menuLink. When you manually execute the statement in the browser console, you're doing so after the entire page has been rendered and menuLink is now in the page context. You will probably find that this script is executing before the DOM is fully loaded even though you've got it at the end of the VF markup. (What tags did you use to do this?

If you're using

Salesforce documentation about accessing the Id as it was rendered in the DOM.

Best Practices for Accessing Component IDs

You don't mention where this ui.js file is stored but depending on the answer to that question, this answer may need to be adjusted to pass references rather than using merge field syntax (which only works within the VF page itself).

If you need to get the generated unique id for the node on the page, you should use the {!$Component.yourTagID} merge field syntax to do so. This mechanism will give you the full id attribute of the element in the page as it was rendered in the browser.

If the path to the component cannot be found as you wrote it, your path is likely incorrect or ambiguous in the page context. It will not throw an error when this happens, it will just return an empty string. i.e. var myVar = ''; where you expected var myVar = 'j_id0:j_id2:myThing'; You can verify what's happening by looking at the source in the browser.

For instance:

      (function (window, document) { var layout = document.getElementById('{!$Component.layout}'); var menu = document.getElementById('{!$Component.menu}'); var menuLink = document.getElementById('{!$Component.menuLink}');

or something more targeted to a specific path from the to the element itself:

      (function (window, document) { var layout = document.getElementById('{!$Component.myPage.myForm.myPageBlock.layout}'); var menu = document.getElementById('{!$Component.myPage.myForm.myPageBlock.menu}'); var menuLink = document.getElementById('{!$Component.myPage.myForm.myPageBlock.menuLink}');

or if you want to pass the DOM id as a function param:

      (function (window, document, layoutId, menuId, menuLinkId) { var layout = document.getElementById(layoutId); var menu = document.getElementById(menuId); var menuLink = document.getElementById(menuLinkId); // check for nulls // content trimmed for brevity }(this, this.document, '{!$Component.layout}', '{!$Component.menu}', '{!$Component.menuLink}'

Another solution to resolve this issue is to load JS files directly at the bottom of the page, and the second one is wrapped by window onload().