Static analysis to eliminate dead code

[jknight12882]

webpack is able to do static analysis on the following UMD code to remove it while rollup leaves it behind.

  // UMD wrapper.
  /*global define:false*/
  if (typeof exports === "object" && typeof module === "object") {
    // CommonJS
    module.exports = _lload;

  } else if (typeof define === "function" && define.amd) {
    // AMD
    define([], function () { return _lload; });

  } else {
    // VanillaJS
    window._lload = _lload;
  }

this should be replace by

if (true) {
    // CommonJS
    module.exports = _lload;
    ...

which can then be eliminated

[Rich-Harris]

This is a two-parter – first, the commonjs plugin would need to rewrite the typeof expressions. Just opened rollup/rollup-plugin-commonjs#151 for that.

Second, Rollup's dead code elimination would need to handle logical expressions in if block conditions. I thought it did, but apparently not – this works, but this doesn't.

[Rich-Harris]

Fixed in 0.38.3 and rollup-plugin-commonjs@7.0.0

[kzc]

Great stuff.

I guess the next logical step would be to add another bit of functionality to remove curly braces around blocks without let and const:

{
	console.log( 'true' );
}

Or perhaps just leave that to the downstream minifier to deal with.

[Rich-Harris]

Yeah, I think Rollup should ideally remove the braces. Minifiers will remove them, so it's not a super high priority, but it does look weird if they're not removed. Not just let and const, I'm fairly sure class and function declarations are also block-scoped.

[kzc]

So just a matter of descending the block AST and if those 4 constructs are not found then the braces can be removed?

[Rich-Harris]

Think so yeah. Wouldn't even need to walk the BlockStatement since any declarations would have to be direct descendants of it, I think, so off the top of my head:

const blockScopedDeclarations = block.children.filter( node =>
  /Declaration/.test( node.type ) && node.kind !== 'var' );

if ( blockScopedDeclarations.length === 0 ) {
  // remove curlies. for extra credit, deindent...
}