I'm testing all the scenarios on reading files using new FileReader(); When the read is done, onload() should be triggered. I have no idea how to trigger onerror() callback Any idea?

        var promise = Promise.create();
        var fileReader = new FileReader();
        fileReader.onload = function () {
        };
        fileReader.onerror = function(event) {
            // I want to stay here ..........
            // Who can tell me how to trigger this onerror callback?
            debugger
        };
        fileReader.readAsText(inputFile);
        return promise;

I'm testing all the scenarios on reading files using new FileReader(); When the read is done, onload() should be triggered. I have no idea how to trigger onerror() callback Any idea?

        var promise = Promise.create();
        var fileReader = new FileReader();
        fileReader.onload = function () {
        };
        fileReader.onerror = function(event) {
            // I want to stay here ..........
            // Who can tell me how to trigger this onerror callback?
            debugger
        };
        fileReader.readAsText(inputFile);
        return promise;

• javascript

• Maybe permissions? – Dennis Vash Commented Sep 19, 2019 at 21:39
• You have to mock the FileReader. If you tell me which libraries are you using for testing maybe I can help you- – Marcos Luis Delgado Commented Sep 19, 2019 at 21:41
• I'm coding on the REAL code. Just want have a quick test for now. Is it possible to trigger this without any testing lib or mock? – newBike Commented Sep 19, 2019 at 21:44

3 Answers 3

Try simply calling abort:

var promise = Promise.create();
var fileReader = new FileReader();
fileReader.onload = function () {
  fileReader.abort()
};
fileReader.onerror = function(event) {
  // I want to stay here ..........
  // Who can tell me how to trigger this onerror callback?
  debugger
};
fileReader.readAsText(inputFile);
return promise;

If that doesn't work you can try a permissions error with something like:

touch testfile
chmod 000 testfile

And open that file from your reader.

Maybe what you're trying to find out is how to use Async functions:

async function FileReader(){
    // your code for the file reading
  return;
};

then use await in your onload() function to wait for it to return:

async function onload(){
  await firstFunction();  // This waits for fileReader() function to finish...
  // do what you want to show when fileReading is done
};

Hope this helps!

You can run your code in debugger mode and add a breakpoint at line "fileReader.readAsText(inputFile)". When the debugger stops, you rename manually the file in your filesystem. FileReader then throws an error. The accepted answer doesn't work in my environment.