How to upload a local (iOS filesystem) file to S3 using React Native with temporary credentials

example.js

/**
 * I recently needed to upload a file from the phone's filesystem to S3 using temporary credentials
 * (i.e. access key, secret key and session token) issued by an API for a React Native application,
 * without the overhead of Base64 encoding the file and passing it over the bridge (as it could be 
 * several MB big), and wanted to avoid writing native code to do this.
 *
 * None of the S3 libraries online worked with the temporary credentials, but I figured out a
 * solution using the official AWS SDK (which is good, as it supports all the relevant authentication
 * out of the box) and the react-native-fetch-blob module, which allows you to upload directly from the
 * filesystem with the correct Content-type header (as far as I can tell, React Native's fetch only
 * allows multipart/form-data, which was causing the pre-signed URL signature check to fail).
 *
 * Below is a skeleton version, which requires the aws-sdk and react-native-fetch-blob npm modules.
 */

import AWS from 'aws-sdk/dist/aws-sdk-react-native';
import RNFetchBlob from 'react-native-fetch-blob'

const uploadFile = async (localPath, contentType, region, accessKeyId, secretAccessKey, sessionToken, bucket, key) => {
  try {
    // Configure the AWS SDK with the credentials
    AWS.config.update({
      region,
      accessKeyId,
      secretAccessKey,
      sessionToken,
    });

    // Create a new instance of the S3 API
    const s3 = new AWS.S3();

    const s3Url = await s3.getSignedUrl('putObject', {
      Bucket: bucket,
      Key: key,
      ContentType: contentType,
    });

    const result = await RNFetchBlob.fetch('PUT', s3Url, {
      'Content-Type': contentType
    }, RNFetchBlob.wrap(localPath));
  } catch (e) {
    console.error('Error', e);
  }
}