Lowering for ES5?

[chowey]

This is an awesome project, by the way!

Do you have any longer-term plans to support ES5? I ask, because this is the only thing preventing me from using esbuild exclusively for all my transpiling needs.

Specifically, I am still required to support IE11, because it still has official support from Microsoft:

Is Internet Explorer 11 the last version of Internet Explorer?

Yes, Internet Explorer 11 is the last major version of Internet Explorer. Internet Explorer 11 will continue receiving security updates and technical support for the lifecycle of the version of Windows on which it is installed. IE 11 is available via Microsoft Update Catalog, Windows Update and Windows Server Update Service (WSUS).

What's worse, the "version of Windows on which it is installed" is Windows 10, which is planned to be the last version of Windows. So IE 11 may not be going away any time soon.

Something like shimport can be used to get ES modules running on IE11, so the only thing missing is ES5 lowering.

Perhaps this is something the community can help with?

[dmichon-msft]

Even without the ability to transpile down to ES5, having the ability to not introduce ES6 code during minification would make esbuild suitable for usage as a replacement for terser on existing ES5-compatible code, which appears to already be a significant performance win. I know that for many projects minification is a significant portion of the ship build time.

I authored this to try to cut down on the time spent in the minification process in large projects; would love to be able to use esbuild as an alternative minifier implementation for even better performance, but I'm stuck needing ES5 support for the time being.

[evanw]

Even without the ability to transpile down to ES5, having the ability to not introduce ES6 code during minification would make esbuild suitable for usage as a replacement for terser on existing ES5-compatible code

This will only happen if you set the target to es6 or above. If you set the target to es5, esbuild will not introduce ES6 code during minification. That flag won't translate ES6 constructs to ES5 because esbuild doesn't have support for that yet, but it will keep ES5 code as ES5 code. Please let me know if you are encountering any errors with ES5 code that unexpectedly becomes ES6 code when the language target is set to es5.

[dmichon-msft]

@evanw, good to know, I guess I misunderstood the issue around arrow functions, since converting function declarations to arrow functions is a pretty common transform when targeting es6. Might be worth mentioning in the docs that you can set the target below es2015, it just won't perform any additional syntax transformations?

[danickinator]

Please let me know if you are encountering any errors with ES5 code that unexpectedly becomes ES6 code when the language target is set to es5.

@evanw the issue that we've come up against trying to target IE11 (we're also locked into supporting it for the foreseeable future) is that IE11 supports a couple of ES6 features - most notably let and const - which will raise an error if esbuild is set to target 'es5'. Unfortunately, targetting 'es6' very quickly generates template literals which IE11 doesn't support.

As a result there currently isn't a target that can safely used to minify code for IE11. An 'ie11' target that was essentially just 'es5' + let/const would be a godsend.

[tooolbox]

Just have to comment how incredibly exciting it is to see the work on lowering template literals to string addition. Three cheers for @evanw !

[HugoDF]

For anyone else like me who was trying to get transpilation to ES5, I've used swc successfully as per this comment: #182 (comment) (thanks @evanw ). It's a magnitude faster than Babel.

[mindplay-dk]

As a result there currently isn't a target that can safely used to minify code for IE11. An 'ie11' target that was essentially just 'es5' + let/const would be a godsend.

While fully supporting ES5 is probably considerably more work, this sounds like relatively low-hanging fruit?

This might be a good, preliminary work-around for the "corporate world", where (sadly) IE11 is still the lowest common denominator.

[chowey]

Reversely, adding support for const/let => var is probably the most straightforward lowering that you could ask for.

If it is a comparable amount of work to do that replacement as it is to add a special "ie11" target, then I vote to just do the replacement (s/const|let/var/g or equivalent).

[mindplay-dk]

@chowey is it that simple? let and const have different scope.

[chowey]

You are right, a straight replacement won't work. Here is an example using Babel:

// input
var a, b;
{
  const a = 1;
  var b = 2;
}

// output
"use strict";

var a, b;
{
  var _a = 1;
  var b = 2;
}

[mindplay-dk]

Hm, there's probably a ton of other features required for IE11 too though?

for..of, template literals, default values for parameters, all the new object literal shorthands...

[tooolbox]

Well, template literals are done 😉