A fast Base64 decoder with a low level API. If you want a high level API, look at base64-js.
npm install --save fast-base64-decode
import base64Decode from 'fast-base64-decode'
// You need to know the length of the decoded data
const result = new Uint8Array(13)
// Pass the string to decode, and a `UInt8Array` where the bytes will be written
base64Decode('SGVsbG8sIFdvcmxkIQ==', result)