Why Does My Userscript\'s onclick Attribute Cause an \'Uncaught ReferenceError: function is not defined\'?

Unresolved ReferenceError: function is undefined with onclick

Problem

In a userscript, an attempt to call a function via an onclick attribute results in an Uncaught ReferenceError: function is not defined.

Investigation

Traditional onclick attribute behavior, where the function is defined in the script, does not work in userscripts due to sandboxing mechanisms.

Solution

To resolve this issue, always use addEventListener() instead of the onclick attribute.

Code Refactor

Replace the onclick attribute with:

document.getElementById("ElementID").addEventListener("click", functionName, false);

Additional Concerns

DOM Overwrites and Event Handlers

Avoid using innerHTML or outerHTML to overwrite DOM elements multiple times, as it removes existing event handlers.

Unique IDs

Ensure that each element has a unique ID. Reusing IDs creates validation issues.

Example

In this code, the onclick attribute has been replaced with addEventListener() and the DOM overwritten issue has been addressed:

for (i = 0; i < EmoteURLLines.length; i++) {
    if (checkIMG (EmoteURLLines[i])) {
        emoteTab[2].innerHTML += '<span>

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