Use polyfills

Transpiling your code is not quite enough to ensure legacy compatibility. Polyfills are the second step to ensuring legacy support.

Here are the recommended polyfills:

core-js to polyfill core/common native features: Promise, URL, Object.assign, array methods, and others.
regenerator-runtime to polyfill native generator functions.
whatwg-fetch to polyfill native fetch.
fast-text-encoding to polyfill native TextEncoder.

NPM install example:

npm install core-js regenerator-runtime whatwg-fetch fast-text-encoding