Comment by fwlr

True for this decade, but in the previous decade it was very much the opposite. Before you used any kind of browser api or nice language feature you would feature-detect it:

    if (typeof Array.prototype.includes === ‘undefined’) { …

And if it wasn’t there you would define it yourself, it was called “polyfilling”. This was so commonplace that we built significant tooling like babel to standardize feature detection tests and fallback implementations - for a few years you could write

    request.then(response => response.json())

And behind the scenes the Rube Goldberg machine would turn this into something that would run in a JavaScript environment that had neither arrow functions nor promises.