Create function that reads an JSON-parses a File object

src/shared/read-json-file.ts

+import { promiseWithResolvers } from './promise-with-resolvers';
+
+/**
+ * Reads and tries to JSON-parse a file
+ * @return - Promise that resolves to parsed content or rejects with any error
+ *           that occurred during parsing
+ */
+export function readJSONFile(
+  file: File,
+  /* istanbul ignore next - test seam */
+  fileReader = new FileReader(),
+): Promise<unknown> {
+  const { promise, resolve, reject } = promiseWithResolvers();
+
+  fileReader.addEventListener('loadend', (e: ProgressEvent<FileReader>) => {
+    try {
+      const content = e.target?.result?.toString() ?? '';
+      const parsed = JSON.parse(content);
+
+      resolve(parsed);
+    } catch (error) {
+      reject(error);
+    }
+  });
+  fileReader.readAsText(file);
+
+  return promise;
+}

src/shared/test/read-json-file-test.js

+import { readJSONFile } from '../read-json-file';
+
+describe('readJSONFile', () => {
+  const file = new File([], 'my-file');
+
+  function createFakeFileReader(fileContent) {
+    return {
+      addEventListener: sinon.stub().callsArgWith(1, {
+        target: {
+          result: fileContent,
+        },
+      }),
+      readAsText: sinon.stub(),
+    };
+  }
+
+  it('rejects when content is not valid JSON', async () => {
+    const fakeFileReader = createFakeFileReader('not JSON');
+
+    await assert.rejects(
+      readJSONFile(file, fakeFileReader),
+      'Unexpected token \'o\', "not JSON" is not valid JSON',
+    );
+    assert.calledWith(fakeFileReader.readAsText, file);
+    assert.calledWith(
+      fakeFileReader.addEventListener,
+      'loadend',
+      sinon.match.func,
+    );
+  });
+
+  it('resolves when content is valid JSON', async () => {
+    const fakeFileReader = createFakeFileReader('{ "foo": "bar" }');
+
+    const content = await readJSONFile(file, fakeFileReader);
+
+    assert.deepEqual(content, { foo: 'bar' });
+    assert.calledWith(fakeFileReader.readAsText, file);
+    assert.calledWith(
+      fakeFileReader.addEventListener,
+      'loadend',
+      sinon.match.func,
+    );
+  });
+});