ES5 compatibility in ES2015 module build

[tommikaikkonen]

Using Webpack 2 to compile an application using most, the minification step generates this error:

ERROR in my_app.js from UglifyJs
SyntaxError: Unexpected token: name (r) [./~/most/src/source/fromIterable.js:22,0][my_app.js:6294,6]

function runProducer (t, iterator, sink) {
  let r = iterator.next()
      ^
  // ...
}

The generated bundle would also fail in browsers that don't support ES6 because of the let keyword.

Webpack 2 uses the value of the module key in package.json to resolve the most package with ES2015 modules, so it can apply tree shaking. In most, this is just the plain source code in most/src.

In the package module points to, all other statements than import and export should be usable in the environments most wants to support. To support ES5, we need to use var instead of let here. This was the only use of let I found in the source code, other than a couple ones in the tests which doesn't matter for app bundle compilation.

This pull request changes that one let to var.

If you guys are planning to use more ES6 features in the codebase, a separate build is probably needed that transpiles to ES5 (or whatever most wants to support), but doesn't transpile the import and export statements so that bundlers can take advantage of tree shaking.

[TylorS]

Hello @tommikaikkonen, thank you very much for your PR! We're working very hard on the next version of mostjs over at @most/core and this was a backport of a much needed improvement which over there is transpiled and it slipped past us here :smile:

[briancavalier]

@tommikaikkonen We just published 1.2.1, with your change. Sorry about that accidental let, and thanks for the fix!

[tommikaikkonen]

Sweet, thanks guys! 👍