parcel-bundler/lib/scope-hoisting/shake.js

"use strict";

const t = require('@babel/types');

const EXPORTS_RE = /^\$([^$]+)\$exports$/;
/**
 * This is a small small implementation of dead code removal specialized to handle
 * removing unused exports. All other dead code removal happens in workers on each
 * individual file by babel-minify.
 */

function treeShake(scope) {
  // Keep passing over all bindings in the scope until we don't remove any.
  // This handles cases where we remove one binding which had a reference to
  // another one. That one will get removed in the next pass if it is now unreferenced.
  let removed;

  do {
    removed = false; // Recrawl to get all bindings.

    scope.crawl();
    Object.keys(scope.bindings).forEach(name => {
      let binding = getUnusedBinding(scope.path, name); // If it is not safe to remove the binding don't touch it.

      if (!binding) {
        return;
      } // Remove the binding and all references to it.


      binding.path.remove();
      binding.referencePaths.concat(binding.constantViolations).forEach(remove);
      scope.removeBinding(name);
      removed = true;
    });
  } while (removed);
}

module.exports = treeShake; // Check if a binding is safe to remove and returns it if it is.

function getUnusedBinding(path, name) {
  let binding = path.scope.getBinding(name);

  if (!binding) {
    return null;
  }

  if (isPure(binding)) {
    return binding;
  }

  if (!EXPORTS_RE.test(name)) {
    return null;
  } // Is there any references which aren't simple assignments?


  let bailout = binding.referencePaths.some(path => !isExportAssignment(path) && !isUnusedWildcard(path));

  if (bailout) {
    return null;
  } else {
    return binding;
  }
}

function isPure(binding) {
  if (binding.referenced) {
    return false;
  }

  if (binding.path.isVariableDeclarator() && binding.path.get('id').isIdentifier()) {
    let init = binding.path.get('init');
    return init.isPure() || init.isIdentifier() || init.isThisExpression();
  }

  return binding.path.isPure();
}

function isExportAssignment(path) {
  return (// match "path.any = any;"
    path.parentPath.isMemberExpression() && path.parentPath.parentPath.isAssignmentExpression() && path.parentPath.parentPath.node.left === path.parentPath.node
  );
}

function isUnusedWildcard(path) {
  let parent = path.parent;
  return (// match `$parcel$exportWildcard` calls
    t.isCallExpression(parent) && t.isIdentifier(parent.callee, {
      name: '$parcel$exportWildcard'
    }) && parent.arguments[0] === path.node && // check if the $id$exports variable is used
    !getUnusedBinding(path, parent.arguments[1].name)
  );
}

function remove(path) {
  if (path.isAssignmentExpression()) {
    if (path.parentPath.isSequenceExpression()) {
      if (path.parent.expressions.length == 1) {
        // replace sequence expression with it's sole child
        path.parentPath.replaceWith(path);
        remove(path.parentPath);
      } else {
        path.remove();
      }
    } else if (!path.parentPath.isExpressionStatement()) {
      path.replaceWith(path.node.right);
    } else {
      path.remove();
    }
  } else if (isExportAssignment(path)) {
    remove(path.parentPath.parentPath);
  } else if (isUnusedWildcard(path)) {
    remove(path.parentPath);
  } else if (!path.removed) {
    path.remove();
  }
}