Deprecate `--target es5`, make lowest target `es2015`

[DanielRosenwasser]

Overview

With TypeScript 7.0, we have no plans to port over our downleveling from ES2015/ES6 to ES5. ECMAScript 5 was a stable and broad target that seemed like the safest option years ago, but the need to support ES5 environments has shrunk dramatically. Additionally, there is quite a bit of complexity in supporting the transformation of generators.

ES2015 is 10 years old at this point, and is a reasonable lowest target.

Because TypeScript 7.0 will not support --target es5, TypeScript 6.0 will be deprecating it.

That means that TypeScript 6.0 will allow you to use --target es5, but you will receive an error that can only be silenced via ignoreDeprecations.

Users who do rely on ES5 runtimes will be able to use older versions of TypeScript for its downlevel compilation capabilities, Babel, or another compiler. But we do encourage teams to re-evaluate whether they need to support ES5 runtimes.

lib, lib.d.ts, and the DOM libraries

Today, lib.d.ts (TypeScript's nickname for its family of built-in library files) is broken up into many files like lib.es5.d.ts, etc.

Changing the default minimum target means that we need to reconsider what default library files get included based on the target, and how lib continues to function. One complication here is the interplay between whether dom includes dom.iterable, or vice versa.

Most recently, our thinking here is:

• es5 continues to exist as a lib target so that it is not an error to reference
• the es2015 lib options automatically includes es5
• the contents of dom.iterable effectively gets included as part of using dom
• dom.iterable will become empty so that it is not an error to reference

Some experimentation around this is taking place at #62025

--downlevelIteration

--downlevelIteration controls whether or not the JS "iterator protocol" is respected (i.e. using Symbol.iterator and the like) by constructs like array spreads, for/of loops, etc. Otherwise, TypeScript would downlevel these constructs in a naive way (e.g. concat and for loops with counter variables).

But this functionality is just assumed to work in ECMAScript 2015, and the flag does nothing in --target es2015. We may want to issue an error message if this flag is specified in conjunction with any newer target, but it seems unnecessary to do so.

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See more deprecation discussions at #54500.

[jakebailey]

An aspect of this is that dom.iterable being moved into dom is that dom will reference lib.es2015.d.ts, and therefore our default lib (which includes DOM) will also be raised. Which, I think we already also want. But it also means that we are explicitly giving up the ability to check DOM with just es5, which, fine.

[guilhermesimoes]

True legacy environments (ES5) are vanishingly rare

Hello, I'd just like to give a different perspective on this. In the world today there are still many devices that only support ES5. For example:

• 117 million PS4s, based on Chromium v44
• 350 million smart TVs pre-2018, with Samsung based on Chromium <v47 and LG based on Chromium <v38
• Millions of set-top boxes that vary by country and ISP

We at PeacockTV, SkyShowtime and NOW care deeply about supporting this older hardware, even if it was released more than 10 years ago, because it's what our users own.

Admittedly this is probably niche — I don't know if there are many companies or devs developing apps for TVs and consoles. But for those of us who are "stuck in the past", taking away --target es5 will necessarily require us to stay on TypeScript 6.0 and forego all the goodies and improvements of TypeScript 7.0 (which we are very much looking forward to).

[jakebailey]

Why can't you run your output through babel to lower to ES5?

[snarbles2]

Might as well ask why you can't just use an older version of typescript.

You can. Either way, it's just a more complicated, less pleasant solution. I don't expect ES5 support to be kept. There's a cost to it. But there is a non-zero upside to it as well.

[RyanCavanaugh]

The statement being responded to there was "we'll have to forego all the goodies and improvements of TypeScript 7.0", which implies an inability to use an alternative downleveler.

[snarbles2]

Call me charitable, I guess. To me it's an imperfectly conveyed sentiment that TypeScript 7 (alone, as-is) will not be suitable for their needs, not that someone doesn't know what babel is or else refuses to consider it. I hope we can all agree that (1) yes, Babel exists and (2) that nonetheless the value of es5 support is non-zero but understandably small.