gistfile1.txt

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// This helps to determine what browser platform this code is being run on. It is detected at runtime.

const BrowserType = function() {
	return {
		NONE: -1,
		Chrome: 0,
		Firefox: 1,
		Edge: 2,
		Get: function() {
			//Isolated namespace checking when we can't even access the background script.
			var result = BrowserType.NONE;
			if (typeof browser === "undefined") {
				result = BrowserType.Chrome;
				return result;
			}
			let supportsPromises = false;
			try {
				supportsPromises = browser.runtime.getPlatformInfo() instanceof Promise;
			}
			catch (e) {
				supportsPromises = false;
			}
			if (!supportsPromises)
				result = BrowserType.Edge;
			else
				result = BrowserType.Firefox;
			return result;
		}
	};
}();
window.namespace = (BrowserType.Get() === BrowserType.Chrome ? chrome : browser);



--------------------------------------


// Arbitrary callback functions scattered throughout the project.
function foo(myFlag, myCondition, myValues) {
  //...
}
function bar(myValues) {
  //...
}

// These arbitrary callback functions are to be wrapped by a wrapper function, given below, to do tasks. Code is wrapped like so:
wrapper_legacy(foo);
wrapper_new(bar);
wrapper_legacy(bar);
wrapper_new(foo);
// etc...

--------------------------------------


// Old code looks like this. We can't modify this code too drastically.
function wrapper_legacy(callback) {
  //A global namespace for Firefox and Edge
  if (BrowserType.Get() === BrowserType.Firefox) {
    window.namespace.tabs.sendMessage({}).then(callback);
  }
  //A global namespace for Chrome
  else if ((BrowserType.Get() === BrowserType.Chrome) || (BrowserType.Get() === BrowserType.Edge)) {
    window.namespace.tabs.sendMessage({}, callback);
  }
}


//Intended new code
function wrapper_new(callback) {
  //Major change - We need to add "chrome.runtime.lastError" specifically for Chrome platform.

  //A global namespace for Firefox and Edge
  if (BrowserType.Get() === BrowserType.Firefox) {
    window.namespace.tabs.sendMessage({}).then(callback);
  }
  //A global namespace for Chrome
  else if ((BrowserType.Get() === BrowserType.Chrome) || (BrowserType.Get() === BrowserType.Edge)) {

    // This is not going to work, because this code needs to be inside the "callback" function called by "tabs.sendMessage", in order to get the runtime error message.
    if (BrowserType.Get() === BrowserType.Chrome)
      if (chrome.runtime.lastError)
        console.log(chrome.runtime.lastError.message);

    window.namespace.tabs.sendMessage({}, callback);
  }
}