How to test FileReader wrapped inside promise? - testing

Question

Current code:
export const readFilePromise = file => {
return new Promise((resolve, reject) => {
let reader = new FileReader();
reader.readAsDataURL(file);
reader.onload = () => {
resolve(reader.result);
};
reader.onerror = error => {
reject(error);
};
});
};
I am able to test the onload method
it('readFilePromise method', async () => {
const fileContentsEncodedInHex = [
'\x45\x6e\x63\x6f\x64\x65\x49\x6e\x48\x65\x78\x42\x65\x63\x61\x75\x73\x65\x42\x69\x6e\x61\x72\x79\x46\x69\x6c\x65\x73\x43\x6f\x6e\x74\x61\x69\x6e\x55\x6e\x70\x72\x69\x6e\x74\x61\x62\x6c\x65\x43\x68\x61\x72\x61\x63\x74\x65\x72\x73'
];
const blob = new Blob(fileContentsEncodedInHex);
const result = readFilePromise(blob);
const output = await result;
const expectedOutput =
'data:;base64,RW5jb2RlSW5IZXhCZWNhdXNlQmluYXJ5RmlsZXNDb250YWluVW5wcmludGFibGVDaGFyYWN0ZXJz';
expect(output).toEqual(expectedOutput);
});
I wonder how do I test the onerror?

Answer 1

If you had something like below because of this
reader.onerror = () => {
reject(reader.error);
};
Then the below should work.
Just know beforehand error message for passing empty file or nothing. I just wanted to create a scenario
describe('Error Case', () => {
it('should return error message for empty file', () => {
const result = readFilePromise();
const n = () => {};
return result.then(n, (errorMessage) => {
expect('your error message here').to.equal(errorMessage);
});
});