JSVL

JSVL, which stands for JavaScript Variable Locator, is a simple tool that allows you to execute code when a global variable is located.

Usage

The main function in JSVL is called as follows:

waitForGlobal(obj, keyPath, callback);

• obj is the object the key is located in. This is usually window, but a userscript might use unsafeWindow (or some abbreviation) when trying to access the contents of the site it's running in.
• keyPath is a string that represents the path to the key. If you're looking for window.foo, keyPath would be "foo". If you're looking for window.foo.bar, keyPath would be "foo.bar".
• callback is the function that's called when the variable you're looking for is found.

Example

This is an example from my Noteflight Tuplet Improver userscript:

var nfeditor;
waitForGlobal(uwin, "nfeditor", () => {
  nfeditor = uwin.nfeditor;
  nfeditor.palette().currentPalette().applyTuplet = (e) => {
    if (e == "septuplet") {
      var l = uwin.prompt("Enter number of notes in tuplet:");
      nfeditor.documentController.controller.createTuplet(parseInt(l));
    } else {
      nfeditor.palette().currentPalette().applyAction("tuplet", {
        duplet: 2,
        triplet: 3,
        quadruplet: 4,
        quintuplet: 5,
        sextuplet: 6
      }[e]);
    }
  };
});

In this example, I created an empty variable called nfeditor, which is the name of the object Noteflight uses to store editor information and functions, like palette().currentPalette().applyTuplet, which inserts a tuplet of an integer length greater than 2 on the selected note or rest (I'm not sure how accurate that was, I'm not a Noteflight developer, after all). I then called waitForGlobal();, where uwin was my variable that represent the Tampermonkey object unsafeWindow, "nfeditor" was the name of the object I was trying to retrieve, and the callback was a function that set my nfeditor variable equal to uwin.nfeditor and started waiting for the user to click the septuplet option in the opened the prompt to enter the number of notes in the tuplet. The callback only gets called when nfeditor exists.