index.js

import type NodePath from "babel-traverse";
import type Scope from "babel-traverse";
import type File from "../../../file";
import traverse from "babel-traverse";
import { visitor as tdzVisitor } from "./tdz";
import * as t from "babel-types";
import values from "lodash/values";
import extend from "lodash/extend";
import template from "babel-template";

export default function () {
  return {
    visitor: {
      VariableDeclaration(path, file) {
        const { node, parent, scope } = path;
        if (!isBlockScoped(node)) return;
        convertBlockScopedToVar(path, null, parent, scope, true);

        if (node._tdzThis) {
          const nodes = [node];

          for (let i = 0; i < node.declarations.length; i++) {
            const decl = node.declarations[i];
            if (decl.init) {
              const assign = t.assignmentExpression("=", decl.id, decl.init);
              assign._ignoreBlockScopingTDZ = true;
              nodes.push(t.expressionStatement(assign));
            }
            decl.init = file.addHelper("temporalUndefined");
          }

          node._blockHoist = 2;

          if (path.isCompletionRecord()) {
            // ensure we don't break completion record semantics by returning
            // the initialiser of the last declarator
            nodes.push(t.expressionStatement(scope.buildUndefinedNode()));
          }

          path.replaceWithMultiple(nodes);
        }
      },

      Loop(path, file) {
        const { node, parent, scope } = path;
        t.ensureBlock(node);
        const blockScoping = new BlockScoping(path, path.get("body"), parent, scope, file);
        const replace = blockScoping.run();
        if (replace) path.replaceWith(replace);
      },

      CatchClause(path, file) {
        const { parent, scope } = path;
        const blockScoping = new BlockScoping(null, path.get("body"), parent, scope, file);
        blockScoping.run();
      },

      "BlockStatement|SwitchStatement|Program"(path, file) {
        if (!ignoreBlock(path)) {
          const blockScoping = new BlockScoping(null, path, path.parent, path.scope, file);
          blockScoping.run();
        }
      }
    }
  };
}

function ignoreBlock(path) {
  return t.isLoop(path.parent) || t.isCatchClause(path.parent);
}

const buildRetCheck = template(`
  if (typeof RETURN === "object") return RETURN.v;
`);

function isBlockScoped(node) {
  if (!t.isVariableDeclaration(node)) return false;
  if (node[t.BLOCK_SCOPED_SYMBOL]) return true;
  if (node.kind !== "let" && node.kind !== "const") return false;
  return true;
}

function convertBlockScopedToVar(path, node, parent, scope, moveBindingsToParent = false) {
  if (!node) {
    node = path.node;
  }
  // https://github.com/babel/babel/issues/255
  if (!t.isFor(parent)) {
    for (let i = 0; i < node.declarations.length; i++) {
      const declar = node.declarations[i];
      declar.init = declar.init || scope.buildUndefinedNode();
    }
  }

  node[t.BLOCK_SCOPED_SYMBOL] = true;
  node.kind = "var";

  // Move bindings from current block scope to function scope.
  if (moveBindingsToParent) {
    const parentScope = scope.getFunctionParent();
    const ids = path.getBindingIdentifiers();
    for (const name in ids) {
      const binding = scope.getOwnBinding(name);
      if (binding) binding.kind = "var";
      scope.moveBindingTo(name, parentScope);
    }
  }
}

function isVar(node) {
  return t.isVariableDeclaration(node, { kind: "var" }) && !isBlockScoped(node);
}

const letReferenceBlockVisitor = traverse.visitors.merge([{
  Loop: {
    enter(path, state) {
      state.loopDepth++;
    },
    exit(path, state) {
      state.loopDepth--;
    },
  },
  Function(path, state) {
    // References to block-scoped variables only require added closures if it's
    // possible for the code to run more than once -- otherwise it is safe to
    // simply rename the variables.
    if (state.loopDepth > 0) {
      path.traverse(letReferenceFunctionVisitor, state);
    }
    return path.skip();
  }
}, tdzVisitor]);

const letReferenceFunctionVisitor = traverse.visitors.merge([{
  ReferencedIdentifier(path, state) {
    const ref = state.letReferences[path.node.name];

    // not a part of our scope
    if (!ref) return;

    // this scope has a variable with the same name so it couldn't belong
    // to our let scope
    const localBinding = path.scope.getBindingIdentifier(path.node.name);
    if (localBinding && localBinding !== ref) return;

    state.closurify = true;
  }
}, tdzVisitor]);

const hoistVarDeclarationsVisitor = {
  enter(path, self) {
    const { node, parent } = path;

    if (path.isForStatement()) {
      if (isVar(node.init, node)) {
        const nodes = self.pushDeclar(node.init);
        if (nodes.length === 1) {
          node.init = nodes[0];
        } else {
          node.init = t.sequenceExpression(nodes);
        }
      }
    } else if (path.isFor()) {
      if (isVar(node.left, node)) {
        self.pushDeclar(node.left);
        node.left = node.left.declarations[0].id;
      }
    } else if (isVar(node, parent)) {
      path.replaceWithMultiple(self.pushDeclar(node).map((expr) => t.expressionStatement(expr)));
    } else if (path.isFunction()) {
      return path.skip();
    }
  }
};

const loopLabelVisitor = {
  LabeledStatement({ node }, state) {
    state.innerLabels.push(node.label.name);
  }
};

const continuationVisitor = {
  enter(path, state) {
    if (path.isAssignmentExpression() || path.isUpdateExpression()) {
      const bindings = path.getBindingIdentifiers();
      for (const name in bindings) {
        if (state.outsideReferences[name] !== path.scope.getBindingIdentifier(name)) continue;
        state.reassignments[name] = true;
      }
    }
  }
};

function loopNodeTo(node) {
  if (t.isBreakStatement(node)) {
    return "break";
  } else if (t.isContinueStatement(node)) {
    return "continue";
  }
}

const loopVisitor = {
  Loop(path, state) {
    const oldIgnoreLabeless = state.ignoreLabeless;
    state.ignoreLabeless = true;
    path.traverse(loopVisitor, state);
    state.ignoreLabeless = oldIgnoreLabeless;
    path.skip();
  },

  Function(path) {
    path.skip();
  },

  SwitchCase(path, state) {
    const oldInSwitchCase = state.inSwitchCase;
    state.inSwitchCase = true;
    path.traverse(loopVisitor, state);
    state.inSwitchCase = oldInSwitchCase;
    path.skip();
  },

  "BreakStatement|ContinueStatement|ReturnStatement"(path, state) {
    const { node, parent, scope } = path;
    if (node[this.LOOP_IGNORE]) return;

    let replace;
    let loopText = loopNodeTo(node);

    if (loopText) {
      if (node.label) {
        // we shouldn't be transforming this because it exists somewhere inside
        if (state.innerLabels.indexOf(node.label.name) >= 0) {
          return;
        }

        loopText = `${loopText}|${node.label.name}`;
      } else {
        // we shouldn't be transforming these statements because
        // they don't refer to the actual loop we're scopifying
        if (state.ignoreLabeless) return;

        //
        if (state.inSwitchCase) return;

        // break statements mean something different in this context
        if (t.isBreakStatement(node) && t.isSwitchCase(parent)) return;
      }

      state.hasBreakContinue = true;
      state.map[loopText] = node;
      replace = t.stringLiteral(loopText);
    }

    if (path.isReturnStatement()) {
      state.hasReturn = true;
      replace = t.objectExpression([
        t.objectProperty(t.identifier("v"), node.argument || scope.buildUndefinedNode())
      ]);
    }

    if (replace) {
      replace = t.returnStatement(replace);
      replace[this.LOOP_IGNORE] = true;
      path.skip();
      path.replaceWith(t.inherits(replace, node));
    }
  }
};

class BlockScoping {
  constructor(loopPath?: NodePath, blockPath: NodePath, parent: Object, scope: Scope, file: File) {
    this.parent = parent;
    this.scope  = scope;
    this.file   = file;

    this.blockPath = blockPath;
    this.block     = blockPath.node;

    this.outsideLetReferences = Object.create(null);
    this.hasLetReferences     = false;
    this.letReferences        = Object.create(null);
    this.body                 = [];

    if (loopPath) {
      this.loopParent = loopPath.parent;
      this.loopLabel  = t.isLabeledStatement(this.loopParent) && this.loopParent.label;
      this.loopPath   = loopPath;
      this.loop       = loopPath.node;
    }
  }

  /**
   * Start the ball rolling.
   */

  run() {
    const block = this.block;
    if (block._letDone) return;
    block._letDone = true;

    const needsClosure = this.getLetReferences();

    // this is a block within a `Function/Program` so we can safely leave it be
    if (t.isFunction(this.parent) || t.isProgram(this.block)) {
      this.updateScopeInfo();
      return;
    }

    // we can skip everything
    if (!this.hasLetReferences) return;

    if (needsClosure) {
      this.wrapClosure();
    } else {
      this.remap();
    }

    this.updateScopeInfo(needsClosure);

    if (this.loopLabel && !t.isLabeledStatement(this.loopParent)) {
      return t.labeledStatement(this.loopLabel, this.loop);
    }
  }

  updateScopeInfo(wrappedInClosure) {
    const scope = this.scope;
    const parentScope = scope.getFunctionParent();
    const letRefs = this.letReferences;

    for (const key in letRefs) {
      const ref = letRefs[key];
      const binding = scope.getBinding(ref.name);
      if (!binding) continue;
      if (binding.kind === "let" || binding.kind === "const") {
        binding.kind = "var";

        if (wrappedInClosure) {
          scope.removeBinding(ref.name);
        } else {
          scope.moveBindingTo(ref.name, parentScope);
        }
      }
    }
  }

  remap() {
    const letRefs   = this.letReferences;
    const scope     = this.scope;

    // alright, so since we aren't wrapping this block in a closure
    // we have to check if any of our let variables collide with
    // those in upper scopes and then if they do, generate a uid
    // for them and replace all references with it

    for (const key in letRefs) {
      // just an Identifier node we collected in `getLetReferences`
      // this is the defining identifier of a declaration
      const ref = letRefs[key];

      // todo: could skip this if the colliding binding is in another function
      if (scope.parentHasBinding(key) || scope.hasGlobal(key)) {
        // The same identifier might have been bound separately in the block scope and
        // the enclosing scope (e.g. loop or catch statement), so we should handle both
        // individually
        if (scope.hasOwnBinding(key))
          scope.rename(ref.name);

        if (this.blockPath.scope.hasOwnBinding(key))
          this.blockPath.scope.rename(ref.name);
      }
    }
  }

  wrapClosure() {
    if (this.file.opts.throwIfClosureRequired) {
      throw this.blockPath.buildCodeFrameError(
        "Compiling let/const in this block would add a closure " +
        "(throwIfClosureRequired)."
      );
    }
    const block = this.block;

    const outsideRefs = this.outsideLetReferences;

    // remap loop heads with colliding variables
    if (this.loop) {
      for (const name in outsideRefs) {
        const id = outsideRefs[name];

        if (this.scope.hasGlobal(id.name) || this.scope.parentHasBinding(id.name)) {
          delete outsideRefs[id.name];
          delete this.letReferences[id.name];

          this.scope.rename(id.name);

          this.letReferences[id.name] = id;
          outsideRefs[id.name] = id;
        }
      }
    }

    // if we're inside of a for loop then we search to see if there are any
    // `break`s, `continue`s, `return`s etc
    this.has = this.checkLoop();

    // hoist let references to retain scope
    this.hoistVarDeclarations();

    // turn outsideLetReferences into an array
    const params = values(outsideRefs);
    const args   = values(outsideRefs);

    const isSwitch = this.blockPath.isSwitchStatement();

    // build the closure that we're going to wrap the block with, possible wrapping switch(){}
    const fn = t.functionExpression(null, params,
      t.blockStatement(isSwitch ? [block] : block.body));
    fn.shadow = true;

    // continuation
    this.addContinuations(fn);

    let ref = fn;

    if (this.loop) {
      ref = this.scope.generateUidIdentifier("loop");
      this.loopPath.insertBefore(t.variableDeclaration("var", [
        t.variableDeclarator(ref, fn)
      ]));
    }

    // build a call and a unique id that we can assign the return value to
    let call = t.callExpression(ref, args);
    const ret  = this.scope.generateUidIdentifier("ret");

    // handle generators
    const hasYield = traverse.hasType(fn.body, this.scope, "YieldExpression", t.FUNCTION_TYPES);
    if (hasYield) {
      fn.generator = true;
      call = t.yieldExpression(call, true);
    }

    // handlers async functions
    const hasAsync = traverse.hasType(fn.body, this.scope, "AwaitExpression", t.FUNCTION_TYPES);
    if (hasAsync) {
      fn.async = true;
      call = t.awaitExpression(call);
    }

    this.buildClosure(ret, call);

    // replace the current block body with the one we're going to build
    if (isSwitch) this.blockPath.replaceWithMultiple(this.body);
    else block.body = this.body;
  }

  /**
   * Push the closure to the body.
   */

  buildClosure(ret: { type: "Identifier" }, call: { type: "CallExpression" }) {
    const has = this.has;
    if (has.hasReturn || has.hasBreakContinue) {
      this.buildHas(ret, call);
    } else {
      this.body.push(t.expressionStatement(call));
    }
  }

  /**
   * If any of the outer let variables are reassigned then we need to rename them in
   * the closure so we can get direct access to the outer variable to continue the
   * iteration with bindings based on each iteration.
   *
   * Reference: https://github.com/babel/babel/issues/1078
   */

  addContinuations(fn) {
    const state = {
      reassignments: {},
      outsideReferences: this.outsideLetReferences
    };

    this.scope.traverse(fn, continuationVisitor, state);

    for (let i = 0; i < fn.params.length; i++) {
      const param = fn.params[i];
      if (!state.reassignments[param.name]) continue;

      const newParam = this.scope.generateUidIdentifier(param.name);
      fn.params[i] = newParam;

      this.scope.rename(param.name, newParam.name, fn);

      // assign outer reference as it's been modified internally and needs to be retained
      fn.body.body.push(t.expressionStatement(t.assignmentExpression("=", param, newParam)));
    }
  }

  getLetReferences() {
    const block = this.block;

    let declarators = [];

    if (this.loop) {
      const init = this.loop.left || this.loop.init;
      if (isBlockScoped(init)) {
        declarators.push(init);
        extend(this.outsideLetReferences, t.getBindingIdentifiers(init));
      }
    }

    const addDeclarationsFromChild = (path, node) => {
      node = node || path.node;
      if (t.isClassDeclaration(node) || t.isFunctionDeclaration(node) || isBlockScoped(node)) {
        if (isBlockScoped(node)) {
          convertBlockScopedToVar(path, node, block, this.scope);
        }
        declarators = declarators.concat(node.declarations || node);
      }
      if (t.isLabeledStatement(node)) {
        addDeclarationsFromChild(path.get("body"), node.body);
      }
    };

    //
    if (block.body) {
      for (let i = 0; i < block.body.length; i++) {
        const declarPath = this.blockPath.get("body")[i];
        addDeclarationsFromChild(declarPath);
      }
    }

    if (block.cases) {
      for (let i = 0; i < block.cases.length; i++) {
        const consequents = block.cases[i].consequent;

        for (let j = 0; j < consequents.length; j++) {
          const declarPath = this.blockPath.get("cases")[i];
          const declar = consequents[j];
          addDeclarationsFromChild(declarPath, declar);
        }
      }
    }

    //
    for (let i = 0; i < declarators.length; i++) {
      const declar = declarators[i];
      // Passing true as the third argument causes t.getBindingIdentifiers
      // to return only the *outer* binding identifiers of this
      // declaration, rather than (for example) mistakenly including the
      // parameters of a function declaration. Fixes #4880.
      const keys = t.getBindingIdentifiers(declar, false, true);
      extend(this.letReferences, keys);
      this.hasLetReferences = true;
    }

    // no let references so we can just quit
    if (!this.hasLetReferences) return;

    const state = {
      letReferences: this.letReferences,
      closurify:     false,
      file:          this.file,
      loopDepth:     0,
    };

    const loopOrFunctionParent = this.blockPath.find(
      (path) => path.isLoop() || path.isFunction()
    );
    if (loopOrFunctionParent && loopOrFunctionParent.isLoop()) {
      // There is a loop ancestor closer than the closest function, so we
      // consider ourselves to be in a loop.
      state.loopDepth++;
    }

    // traverse through this block, stopping on functions and checking if they
    // contain any local let references
    this.blockPath.traverse(letReferenceBlockVisitor, state);

    return state.closurify;
  }

  /**
   * If we're inside of a loop then traverse it and check if it has one of
   * the following node types `ReturnStatement`, `BreakStatement`,
   * `ContinueStatement` and replace it with a return value that we can track
   * later on.
   */

  checkLoop(): Object {
    const state = {
      hasBreakContinue: false,
      ignoreLabeless:   false,
      inSwitchCase:     false,
      innerLabels:      [],
      hasReturn:        false,
      isLoop:           !!this.loop,
      map:              {},
      LOOP_IGNORE:      Symbol()
    };

    this.blockPath.traverse(loopLabelVisitor, state);
    this.blockPath.traverse(loopVisitor, state);

    return state;
  }

  /**
   * Hoist all let declarations in this block to before it so they retain scope
   * once we wrap everything in a closure.
   */

  hoistVarDeclarations() {
    this.blockPath.traverse(hoistVarDeclarationsVisitor, this);
  }

  /**
   * Turn a `VariableDeclaration` into an array of `AssignmentExpressions` with
   * their declarations hoisted to before the closure wrapper.
   */

  pushDeclar(node: { type: "VariableDeclaration" }): Array<Object> {
    const declars = [];
    const names = t.getBindingIdentifiers(node);
    for (const name in names) {
      declars.push(t.variableDeclarator(names[name]));
    }

    this.body.push(t.variableDeclaration(node.kind, declars));

    const replace = [];

    for (let i = 0; i < node.declarations.length; i++) {
      const declar = node.declarations[i];
      if (!declar.init) continue;

      const expr = t.assignmentExpression("=", declar.id, declar.init);
      replace.push(t.inherits(expr, declar));
    }

    return replace;
  }

  buildHas(ret: { type: "Identifier" }, call: { type: "CallExpression" }) {
    const body = this.body;

    body.push(t.variableDeclaration("var", [
      t.variableDeclarator(ret, call)
    ]));

    let retCheck;
    const has = this.has;
    const cases = [];

    if (has.hasReturn) {
      // typeof ret === "object"
      retCheck = buildRetCheck({
        RETURN: ret
      });
    }

    if (has.hasBreakContinue) {
      for (const key in has.map) {
        cases.push(t.switchCase(t.stringLiteral(key), [has.map[key]]));
      }

      if (has.hasReturn) {
        cases.push(t.switchCase(null, [retCheck]));
      }

      if (cases.length === 1) {
        const single = cases[0];
        body.push(t.ifStatement(
          t.binaryExpression("===", ret, single.test),
          single.consequent[0]
        ));
      } else {
        if (this.loop) {
          // https://github.com/babel/babel/issues/998
          for (let i = 0; i < cases.length; i++) {
            const caseConsequent = cases[i].consequent[0];
            if (t.isBreakStatement(caseConsequent) && !caseConsequent.label) {
              caseConsequent.label = this.loopLabel = this.loopLabel ||
                this.scope.generateUidIdentifier("loop");
            }
          }
        }

        body.push(t.switchStatement(ret, cases));
      }
    } else {
      if (has.hasReturn) {
        body.push(retCheck);
      }
    }
  }
}