Function iteration

tools/acorn-optimizer.mjs:219–404

()

function JSDCE(ast, aggressive) {
  function iteration() {
    let removed = 0;
    const scopes = [{}]; // begin with empty toplevel scope
    function ensureData(scope, name) {
      if (Object.prototype.hasOwnProperty.call(scope, name)) return scope[name];
      scope[name] = {
        def: 0,
        use: 0,
        param: 0, // true for function params, which cannot be eliminated
      };
      return scope[name];
    }
    function cleanUp(ast, names) {
      recursiveWalk(ast, {
        ForStatement(node, c) {
          visitChildren(node, c);
          // If we had `for (var x = ...; ...)` and we removed `x`, we need to change to `for (; ...)`.
          if (node.init?.type === 'EmptyStatement') {
            node.init = null;
          }
        },
        ForInStatement(node, c) {
          // We can't remove the var in a for-in, as that would result in an invalid syntax. Skip the LHS.
          c(node.right);
          c(node.body);
        },
        ForOfStatement(node, c) {
          // We can't remove the var in a for-of, as that would result in an invalid syntax. Skip the LHS.
          c(node.right);
          c(node.body);
        },
        VariableDeclaration(node, _c) {
          let removedHere = 0;
          node.declarations = node.declarations.filter((node) => {
            assert(node.type === 'VariableDeclarator');
            let keep = node.init && hasSideEffects(node.init);
            walkPattern(
              node.id,
              (value) => {
                keep ||= hasSideEffects(value);
              },
              (boundName) => {
                keep ||= !names.has(boundName);
              },
            );
            if (!keep) removedHere = 1;
            return keep;
          });
          removed += removedHere;
          if (node.declarations.length === 0) {
            emptyOut(node);
          }
        },
        ExpressionStatement(node, _c) {
          if (aggressive && !hasSideEffects(node)) {
            emptyOut(node);
            removed++;
          }