Concatenate mixed static + dynamic attribute values

[jviide]

This pull request implements string concatenation for mixed static + dynamic attributes and adds related tests. As long as there's more than one dynamic and/or static value they get interpreted as strings and concatenated. In practice this means that <div a="${1}2" />, <div a="1${2}" />, <div a="${1}${2}" />, <div a=${1}${2} /> and so on are effectively parsed as <div a=${"1" + "2"} />.

As a side effect HTM now allows empty attribute names like <div =1 /> or <div ""=1 />.

The htm.mjs.br size changes by +26 bytes (540 B -> 566 B) for the default build and +18 bytes (376 B -> 394 B) for the mini build.

Closes #58.

[jviide]

Marked this PR as draft, because it doesn't yet implement the string concatenation for the Babel plugins.

[jviide]

Now babel-plugin-htm also supports concatenating mixed static + dynamic attribute values. Some refactoring was necessary. The build and evaluate functions are in their own internal module now, along with a new treeify function. treeify takes the output of build and produces a tree-like structure that's hopefully bit easier to analyze and transform without needing to know about the gnarly size optimizations inside build/evaluate. The source code contains a small example how such a tree looks like.

The logic tries to keep the resulting code at least somewhat compact. For example <a b=${1}${2}> transforms to h("a",{b:""+1+2}) because the string coercion is needed. However, <a b=${"1"}${2}> transforms to h("a",{b:"1"+2}), as the "" + part would be unnecessary.

[jviide]

Pushed a commit that fixed a performance regression in the non-mini version. Now the master and this PR get the same results when running npm run test:perf.

As a nice bonus the build size shrunk by 3 bytes, so the in total this branch adds +23 bytes to the non-mini build.

[developit]

Does this get bundled? If not the path will fail when published.

[jviide]

Yes, build and treeify get bundled to dist/babel-plugin-htm.js (and the corresponding .mjs file).

(Fixed the weird tab between { build, and treeify } though 🙂)