add util function for creating base64 string from file in browser env

Author: tsmith023

src/index.ts

@@ -105,3 +105,4 @@ export * from './backup';
 export * from './cluster';
 export * from './connection';
 export * from './utils/uuid';
+export * from './utils/base64';

src/utils/base64.ts

@@ -0,0 +1,46 @@
+/**
+ * This function converts a file blob into a base64 string so that it can be
+ * sent to Weaviate and stored as a media field.
+ *
+ * This specific function is only applicable within the browser since it depends on
+ * the FileReader API. It will throw an error if it is called in a Node environment.
+ *
+ * @param {Blob} blob The file blob to convert
+ * @returns {string} The base64 string
+ * @throws An error if the function is called outside of the browser
+ *
+ * @example
+ * // Vanilla JS
+ * const file = document.querySelector('input[type="file"]').files[0];
+ * toBase64FromBlob(file).then((base64) => console.log(base64));
+ *
+ * // React
+ * const [base64, setBase64] = useState('');
+ * const onChange = (event) => toBase64FromBlob(event.target.files[0]).then(setBase64);
+ *
+ * // Submit
+ * const onSubmit = (base64: string) => client.data
+ *     .creator()
+ *     .withClassName('MyClass')
+ *     .withProperties({ myMediaField: base64 })
+ *     .do();
+ *
+ */
+export function toBase64FromBlob(blob: Blob): Promise<string> {
+  if (typeof window === 'undefined') {
+    throw new Error('This function is only available in the browser');
+  }
+  const reader = new FileReader();
+  return new Promise((resolve, reject) => {
+    reader.onload = resolve;
+    reader.onerror = reject;
+    reader.readAsDataURL(blob);
+  }).then(() => {
+    if (typeof reader.result !== 'string') {
+      throw new Error(
+        `Unexpected result when converting blob to base64 (result is not a string): ${reader.result}`
+      );
+    }
+    return reader.result;
+  });
+}