Determine whether the action exists

This may not be elegant, but it does what you're asking for.

function actionExists(actionSet, actionName) {
    var actionSetExists = false;
    var actionExists = false;

    var actionSetRef = new ActionReference();
    actionSetRef.putName(charIDToTypeID("ASet"), actionSet);

    try {
        var actionSetDesc = executeActionGet(actionSetRef);
        actionSetExists = true;

        var actionRef = new ActionReference();
        actionRef.putName(charIDToTypeID("Actn"), actionName);
        actionRef.putIndex(charIDToTypeID("ASet"), actionSetDesc.getInteger(charIDToTypeID("ItmI")));

        try {
            executeActionGet(actionRef);
            actionExists = true;
        } catch (e) {
            actionExists = false;
        }
    } catch (e) {
        actionSetExists = false;
    }

    return { actionSetExists: actionSetExists, actionExists: actionExists };
}

var actionSet = prompt("Enter the name of the Action Set:");
var actionName = prompt("Enter the name of the Action:");

var result = actionExists(actionSet, actionName);
alert('Action Set: ' + actionSet + ' Exists: ' + result.actionSetExists + '\nAction: ' + actionName + ' Exists: ' + result.actionExists);

 Thank Microsoft Copilot. 🙂