switch to vuepress

1 files changed, 1207 insertions, 0 deletions

diff --git a/node_modules/regenerator-transform/src/emit.js b/node_modules/regenerator-transform/src/emit.js
new file mode 100644
index 00000000..040faec3
--- /dev/null
+++ b/node_modules/regenerator-transform/src/emit.js
@@ -0,0 +1,1207 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import assert from "assert";
+import * as leap from "./leap";
+import * as meta from "./meta";
+import * as util from "./util";
+
+let hasOwn = Object.prototype.hasOwnProperty;
+
+function Emitter(contextId) {
+  assert.ok(this instanceof Emitter);
+
+  util.getTypes().assertIdentifier(contextId);
+
+  // Used to generate unique temporary names.
+  this.nextTempId = 0;
+
+  // In order to make sure the context object does not collide with
+  // anything in the local scope, we might have to rename it, so we
+  // refer to it symbolically instead of just assuming that it will be
+  // called "context".
+  this.contextId = contextId;
+
+  // An append-only list of Statements that grows each time this.emit is
+  // called.
+  this.listing = [];
+
+  // A sparse array whose keys correspond to locations in this.listing
+  // that have been marked as branch/jump targets.
+  this.marked = [true];
+
+  // The last location will be marked when this.getDispatchLoop is
+  // called.
+  this.finalLoc = loc();
+
+  // A list of all leap.TryEntry statements emitted.
+  this.tryEntries = [];
+
+  // Each time we evaluate the body of a loop, we tell this.leapManager
+  // to enter a nested loop context that determines the meaning of break
+  // and continue statements therein.
+  this.leapManager = new leap.LeapManager(this);
+}
+
+let Ep = Emitter.prototype;
+exports.Emitter = Emitter;
+
+// Offsets into this.listing that could be used as targets for branches or
+// jumps are represented as numeric Literal nodes. This representation has
+// the amazingly convenient benefit of allowing the exact value of the
+// location to be determined at any time, even after generating code that
+// refers to the location.
+function loc() {
+  return util.getTypes().numericLiteral(-1);
+}
+
+// Sets the exact value of the given location to the offset of the next
+// Statement emitted.
+Ep.mark = function(loc) {
+  util.getTypes().assertLiteral(loc);
+  let index = this.listing.length;
+  if (loc.value === -1) {
+    loc.value = index;
+  } else {
+    // Locations can be marked redundantly, but their values cannot change
+    // once set the first time.
+    assert.strictEqual(loc.value, index);
+  }
+  this.marked[index] = true;
+  return loc;
+};
+
+Ep.emit = function(node) {
+  const t = util.getTypes();
+
+  if (t.isExpression(node)) {
+    node = t.expressionStatement(node);
+  }
+
+  t.assertStatement(node);
+  this.listing.push(node);
+};
+
+// Shorthand for emitting assignment statements. This will come in handy
+// for assignments to temporary variables.
+Ep.emitAssign = function(lhs, rhs) {
+  this.emit(this.assign(lhs, rhs));
+  return lhs;
+};
+
+// Shorthand for an assignment statement.
+Ep.assign = function(lhs, rhs) {
+  const t = util.getTypes();
+  return t.expressionStatement(
+    t.assignmentExpression("=", lhs, rhs));
+};
+
+// Convenience function for generating expressions like context.next,
+// context.sent, and context.rval.
+Ep.contextProperty = function(name, computed) {
+  const t = util.getTypes();
+  return t.memberExpression(
+    this.contextId,
+    computed ? t.stringLiteral(name) : t.identifier(name),
+    !!computed
+  );
+};
+
+// Shorthand for setting context.rval and jumping to `context.stop()`.
+Ep.stop = function(rval) {
+  if (rval) {
+    this.setReturnValue(rval);
+  }
+
+  this.jump(this.finalLoc);
+};
+
+Ep.setReturnValue = function(valuePath) {
+  util.getTypes().assertExpression(valuePath.value);
+
+  this.emitAssign(
+    this.contextProperty("rval"),
+    this.explodeExpression(valuePath)
+  );
+};
+
+Ep.clearPendingException = function(tryLoc, assignee) {
+  const t = util.getTypes();
+
+  t.assertLiteral(tryLoc);
+
+  let catchCall = t.callExpression(
+    this.contextProperty("catch", true),
+    [tryLoc]
+  );
+
+  if (assignee) {
+    this.emitAssign(assignee, catchCall);
+  } else {
+    this.emit(catchCall);
+  }
+};
+
+// Emits code for an unconditional jump to the given location, even if the
+// exact value of the location is not yet known.
+Ep.jump = function(toLoc) {
+  this.emitAssign(this.contextProperty("next"), toLoc);
+  this.emit(util.getTypes().breakStatement());
+};
+
+// Conditional jump.
+Ep.jumpIf = function(test, toLoc) {
+  const t = util.getTypes();
+
+  t.assertExpression(test);
+  t.assertLiteral(toLoc);
+
+  this.emit(t.ifStatement(
+    test,
+    t.blockStatement([
+      this.assign(this.contextProperty("next"), toLoc),
+      t.breakStatement()
+    ])
+  ));
+};
+
+// Conditional jump, with the condition negated.
+Ep.jumpIfNot = function(test, toLoc) {
+  const t = util.getTypes();
+
+  t.assertExpression(test);
+  t.assertLiteral(toLoc);
+
+  let negatedTest;
+  if (t.isUnaryExpression(test) &&
+      test.operator === "!") {
+    // Avoid double negation.
+    negatedTest = test.argument;
+  } else {
+    negatedTest = t.unaryExpression("!", test);
+  }
+
+  this.emit(t.ifStatement(
+    negatedTest,
+    t.blockStatement([
+      this.assign(this.contextProperty("next"), toLoc),
+      t.breakStatement()
+    ])
+  ));
+};
+
+// Returns a unique MemberExpression that can be used to store and
+// retrieve temporary values. Since the object of the member expression is
+// the context object, which is presumed to coexist peacefully with all
+// other local variables, and since we just increment `nextTempId`
+// monotonically, uniqueness is assured.
+Ep.makeTempVar = function() {
+  return this.contextProperty("t" + this.nextTempId++);
+};
+
+Ep.getContextFunction = function(id) {
+  const t = util.getTypes();
+
+  return t.functionExpression(
+    id || null/*Anonymous*/,
+    [this.contextId],
+    t.blockStatement([this.getDispatchLoop()]),
+    false, // Not a generator anymore!
+    false // Nor an expression.
+  );
+};
+
+// Turns this.listing into a loop of the form
+//
+//   while (1) switch (context.next) {
+//   case 0:
+//   ...
+//   case n:
+//     return context.stop();
+//   }
+//
+// Each marked location in this.listing will correspond to one generated
+// case statement.
+Ep.getDispatchLoop = function() {
+  const self = this;
+  const t = util.getTypes();
+  let cases = [];
+  let current;
+
+  // If we encounter a break, continue, or return statement in a switch
+  // case, we can skip the rest of the statements until the next case.
+  let alreadyEnded = false;
+
+  self.listing.forEach(function(stmt, i) {
+    if (self.marked.hasOwnProperty(i)) {
+      cases.push(t.switchCase(
+        t.numericLiteral(i),
+        current = []));
+      alreadyEnded = false;
+    }
+
+    if (!alreadyEnded) {
+      current.push(stmt);
+      if (t.isCompletionStatement(stmt))
+        alreadyEnded = true;
+    }
+  });
+
+  // Now that we know how many statements there will be in this.listing,
+  // we can finally resolve this.finalLoc.value.
+  this.finalLoc.value = this.listing.length;
+
+  cases.push(
+    t.switchCase(this.finalLoc, [
+      // Intentionally fall through to the "end" case...
+    ]),
+
+    // So that the runtime can jump to the final location without having
+    // to know its offset, we provide the "end" case as a synonym.
+    t.switchCase(t.stringLiteral("end"), [
+      // This will check/clear both context.thrown and context.rval.
+      t.returnStatement(
+        t.callExpression(this.contextProperty("stop"), [])
+      )
+    ])
+  );
+
+  return t.whileStatement(
+    t.numericLiteral(1),
+    t.switchStatement(
+      t.assignmentExpression(
+        "=",
+        this.contextProperty("prev"),
+        this.contextProperty("next")
+      ),
+      cases
+    )
+  );
+};
+
+Ep.getTryLocsList = function() {
+  if (this.tryEntries.length === 0) {
+    // To avoid adding a needless [] to the majority of runtime.wrap
+    // argument lists, force the caller to handle this case specially.
+    return null;
+  }
+
+  const t = util.getTypes();
+  let lastLocValue = 0;
+
+  return t.arrayExpression(
+    this.tryEntries.map(function(tryEntry) {
+      let thisLocValue = tryEntry.firstLoc.value;
+      assert.ok(thisLocValue >= lastLocValue, "try entries out of order");
+      lastLocValue = thisLocValue;
+
+      let ce = tryEntry.catchEntry;
+      let fe = tryEntry.finallyEntry;
+
+      let locs = [
+        tryEntry.firstLoc,
+        // The null here makes a hole in the array.
+        ce ? ce.firstLoc : null
+      ];
+
+      if (fe) {
+        locs[2] = fe.firstLoc;
+        locs[3] = fe.afterLoc;
+      }
+
+      return t.arrayExpression(locs);
+    })
+  );
+};
+
+// All side effects must be realized in order.
+
+// If any subexpression harbors a leap, all subexpressions must be
+// neutered of side effects.
+
+// No destructive modification of AST nodes.
+
+Ep.explode = function(path, ignoreResult) {
+  const t = util.getTypes();
+  let node = path.node;
+  let self = this;
+
+  t.assertNode(node);
+
+  if (t.isDeclaration(node))
+    throw getDeclError(node);
+
+  if (t.isStatement(node))
+    return self.explodeStatement(path);
+
+  if (t.isExpression(node))
+    return self.explodeExpression(path, ignoreResult);
+
+  switch (node.type) {
+  case "Program":
+    return path.get("body").map(
+      self.explodeStatement,
+      self
+    );
+
+  case "VariableDeclarator":
+    throw getDeclError(node);
+
+  // These node types should be handled by their parent nodes
+  // (ObjectExpression, SwitchStatement, and TryStatement, respectively).
+  case "Property":
+  case "SwitchCase":
+  case "CatchClause":
+    throw new Error(
+      node.type + " nodes should be handled by their parents");
+
+  default:
+    throw new Error(
+      "unknown Node of type " +
+        JSON.stringify(node.type));
+  }
+};
+
+function getDeclError(node) {
+  return new Error(
+    "all declarations should have been transformed into " +
+    "assignments before the Exploder began its work: " +
+    JSON.stringify(node));
+}
+
+Ep.explodeStatement = function(path, labelId) {
+  const t = util.getTypes();
+  let stmt = path.node;
+  let self = this;
+  let before, after, head;
+
+  t.assertStatement(stmt);
+
+  if (labelId) {
+    t.assertIdentifier(labelId);
+  } else {
+    labelId = null;
+  }
+
+  // Explode BlockStatement nodes even if they do not contain a yield,
+  // because we don't want or need the curly braces.
+  if (t.isBlockStatement(stmt)) {
+    path.get("body").forEach(function (path) {
+      self.explodeStatement(path);
+    });
+    return;
+  }
+
+  if (!meta.containsLeap(stmt)) {
+    // Technically we should be able to avoid emitting the statement
+    // altogether if !meta.hasSideEffects(stmt), but that leads to
+    // confusing generated code (for instance, `while (true) {}` just
+    // disappears) and is probably a more appropriate job for a dedicated
+    // dead code elimination pass.
+    self.emit(stmt);
+    return;
+  }
+
+  switch (stmt.type) {
+  case "ExpressionStatement":
+    self.explodeExpression(path.get("expression"), true);
+    break;
+
+  case "LabeledStatement":
+    after = loc();
+
+    // Did you know you can break from any labeled block statement or
+    // control structure? Well, you can! Note: when a labeled loop is
+    // encountered, the leap.LabeledEntry created here will immediately
+    // enclose a leap.LoopEntry on the leap manager's stack, and both
+    // entries will have the same label. Though this works just fine, it
+    // may seem a bit redundant. In theory, we could check here to
+    // determine if stmt knows how to handle its own label; for example,
+    // stmt happens to be a WhileStatement and so we know it's going to
+    // establish its own LoopEntry when we explode it (below). Then this
+    // LabeledEntry would be unnecessary. Alternatively, we might be
+    // tempted not to pass stmt.label down into self.explodeStatement,
+    // because we've handled the label here, but that's a mistake because
+    // labeled loops may contain labeled continue statements, which is not
+    // something we can handle in this generic case. All in all, I think a
+    // little redundancy greatly simplifies the logic of this case, since
+    // it's clear that we handle all possible LabeledStatements correctly
+    // here, regardless of whether they interact with the leap manager
+    // themselves. Also remember that labels and break/continue-to-label
+    // statements are rare, and all of this logic happens at transform
+    // time, so it has no additional runtime cost.
+    self.leapManager.withEntry(
+      new leap.LabeledEntry(after, stmt.label),
+      function() {
+        self.explodeStatement(path.get("body"), stmt.label);
+      }
+    );
+
+    self.mark(after);
+
+    break;
+
+  case "WhileStatement":
+    before = loc();
+    after = loc();
+
+    self.mark(before);
+    self.jumpIfNot(self.explodeExpression(path.get("test")), after);
+    self.leapManager.withEntry(
+      new leap.LoopEntry(after, before, labelId),
+      function() { self.explodeStatement(path.get("body")); }
+    );
+    self.jump(before);
+    self.mark(after);
+
+    break;
+
+  case "DoWhileStatement":
+    let first = loc();
+    let test = loc();
+    after = loc();
+
+    self.mark(first);
+    self.leapManager.withEntry(
+      new leap.LoopEntry(after, test, labelId),
+      function() { self.explode(path.get("body")); }
+    );
+    self.mark(test);
+    self.jumpIf(self.explodeExpression(path.get("test")), first);
+    self.mark(after);
+
+    break;
+
+  case "ForStatement":
+    head = loc();
+    let update = loc();
+    after = loc();
+
+    if (stmt.init) {
+      // We pass true here to indicate that if stmt.init is an expression
+      // then we do not care about its result.
+      self.explode(path.get("init"), true);
+    }
+
+    self.mark(head);
+
+    if (stmt.test) {
+      self.jumpIfNot(self.explodeExpression(path.get("test")), after);
+    } else {
+      // No test means continue unconditionally.
+    }
+
+    self.leapManager.withEntry(
+      new leap.LoopEntry(after, update, labelId),
+      function() { self.explodeStatement(path.get("body")); }
+    );
+
+    self.mark(update);
+
+    if (stmt.update) {
+      // We pass true here to indicate that if stmt.update is an
+      // expression then we do not care about its result.
+      self.explode(path.get("update"), true);
+    }
+
+    self.jump(head);
+
+    self.mark(after);
+
+    break;
+
+  case "TypeCastExpression":
+    return self.explodeExpression(path.get("expression"));
+
+  case "ForInStatement":
+    head = loc();
+    after = loc();
+
+    let keyIterNextFn = self.makeTempVar();
+    self.emitAssign(
+      keyIterNextFn,
+      t.callExpression(
+        util.runtimeProperty("keys"),
+        [self.explodeExpression(path.get("right"))]
+      )
+    );
+
+    self.mark(head);
+
+    let keyInfoTmpVar = self.makeTempVar();
+    self.jumpIf(
+      t.memberExpression(
+        t.assignmentExpression(
+          "=",
+          keyInfoTmpVar,
+          t.callExpression(keyIterNextFn, [])
+        ),
+        t.identifier("done"),
+        false
+      ),
+      after
+    );
+
+    self.emitAssign(
+      stmt.left,
+      t.memberExpression(
+        keyInfoTmpVar,
+        t.identifier("value"),
+        false
+      )
+    );
+
+    self.leapManager.withEntry(
+      new leap.LoopEntry(after, head, labelId),
+      function() { self.explodeStatement(path.get("body")); }
+    );
+
+    self.jump(head);
+
+    self.mark(after);
+
+    break;
+
+  case "BreakStatement":
+    self.emitAbruptCompletion({
+      type: "break",
+      target: self.leapManager.getBreakLoc(stmt.label)
+    });
+
+    break;
+
+  case "ContinueStatement":
+    self.emitAbruptCompletion({
+      type: "continue",
+      target: self.leapManager.getContinueLoc(stmt.label)
+    });
+
+    break;
+
+  case "SwitchStatement":
+    // Always save the discriminant into a temporary variable in case the
+    // test expressions overwrite values like context.sent.
+    let disc = self.emitAssign(
+      self.makeTempVar(),
+      self.explodeExpression(path.get("discriminant"))
+    );
+
+    after = loc();
+    let defaultLoc = loc();
+    let condition = defaultLoc;
+    let caseLocs = [];
+
+    // If there are no cases, .cases might be undefined.
+    let cases = stmt.cases || [];
+
+    for (let i = cases.length - 1; i >= 0; --i) {
+      let c = cases[i];
+      t.assertSwitchCase(c);
+
+      if (c.test) {
+        condition = t.conditionalExpression(
+          t.binaryExpression("===", disc, c.test),
+          caseLocs[i] = loc(),
+          condition
+        );
+      } else {
+        caseLocs[i] = defaultLoc;
+      }
+    }
+
+    let discriminant = path.get("discriminant");
+    util.replaceWithOrRemove(discriminant, condition);
+    self.jump(self.explodeExpression(discriminant));
+
+    self.leapManager.withEntry(
+      new leap.SwitchEntry(after),
+      function() {
+        path.get("cases").forEach(function(casePath) {
+          let i = casePath.key;
+          self.mark(caseLocs[i]);
+
+          casePath.get("consequent").forEach(function (path) {
+            self.explodeStatement(path);
+          });
+        });
+      }
+    );
+
+    self.mark(after);
+    if (defaultLoc.value === -1) {
+      self.mark(defaultLoc);
+      assert.strictEqual(after.value, defaultLoc.value);
+    }
+
+    break;
+
+  case "IfStatement":
+    let elseLoc = stmt.alternate && loc();
+    after = loc();
+
+    self.jumpIfNot(
+      self.explodeExpression(path.get("test")),
+      elseLoc || after
+    );
+
+    self.explodeStatement(path.get("consequent"));
+
+    if (elseLoc) {
+      self.jump(after);
+      self.mark(elseLoc);
+      self.explodeStatement(path.get("alternate"));
+    }
+
+    self.mark(after);
+
+    break;
+
+  case "ReturnStatement":
+    self.emitAbruptCompletion({
+      type: "return",
+      value: self.explodeExpression(path.get("argument"))
+    });
+
+    break;
+
+  case "WithStatement":
+    throw new Error("WithStatement not supported in generator functions.");
+
+  case "TryStatement":
+    after = loc();
+
+    let handler = stmt.handler;
+
+    let catchLoc = handler && loc();
+    let catchEntry = catchLoc && new leap.CatchEntry(
+      catchLoc,
+      handler.param
+    );
+
+    let finallyLoc = stmt.finalizer && loc();
+    let finallyEntry = finallyLoc &&
+      new leap.FinallyEntry(finallyLoc, after);
+
+    let tryEntry = new leap.TryEntry(
+      self.getUnmarkedCurrentLoc(),
+      catchEntry,
+      finallyEntry
+    );
+
+    self.tryEntries.push(tryEntry);
+    self.updateContextPrevLoc(tryEntry.firstLoc);
+
+    self.leapManager.withEntry(tryEntry, function() {
+      self.explodeStatement(path.get("block"));
+
+      if (catchLoc) {
+        if (finallyLoc) {
+          // If we have both a catch block and a finally block, then
+          // because we emit the catch block first, we need to jump over
+          // it to the finally block.
+          self.jump(finallyLoc);
+
+        } else {
+          // If there is no finally block, then we need to jump over the
+          // catch block to the fall-through location.
+          self.jump(after);
+        }
+
+        self.updateContextPrevLoc(self.mark(catchLoc));
+
+        let bodyPath = path.get("handler.body");
+        let safeParam = self.makeTempVar();
+        self.clearPendingException(tryEntry.firstLoc, safeParam);
+
+        bodyPath.traverse(catchParamVisitor, {
+          safeParam: safeParam,
+          catchParamName: handler.param.name
+        });
+
+        self.leapManager.withEntry(catchEntry, function() {
+          self.explodeStatement(bodyPath);
+        });
+      }
+
+      if (finallyLoc) {
+        self.updateContextPrevLoc(self.mark(finallyLoc));
+
+        self.leapManager.withEntry(finallyEntry, function() {
+          self.explodeStatement(path.get("finalizer"));
+        });
+
+        self.emit(t.returnStatement(t.callExpression(
+          self.contextProperty("finish"),
+          [finallyEntry.firstLoc]
+        )));
+      }
+    });
+
+    self.mark(after);
+
+    break;
+
+  case "ThrowStatement":
+    self.emit(t.throwStatement(
+      self.explodeExpression(path.get("argument"))
+    ));
+
+    break;
+
+  default:
+    throw new Error(
+      "unknown Statement of type " +
+        JSON.stringify(stmt.type));
+  }
+};
+
+let catchParamVisitor = {
+  Identifier: function(path, state) {
+    if (path.node.name === state.catchParamName && util.isReference(path)) {
+      util.replaceWithOrRemove(path, state.safeParam);
+    }
+  },
+
+  Scope: function(path, state) {
+    if (path.scope.hasOwnBinding(state.catchParamName)) {
+      // Don't descend into nested scopes that shadow the catch
+      // parameter with their own declarations.
+      path.skip();
+    }
+  }
+};
+
+Ep.emitAbruptCompletion = function(record) {
+  if (!isValidCompletion(record)) {
+    assert.ok(
+      false,
+      "invalid completion record: " +
+        JSON.stringify(record)
+    );
+  }
+
+  assert.notStrictEqual(
+    record.type, "normal",
+    "normal completions are not abrupt"
+  );
+
+  const t = util.getTypes();
+  let abruptArgs = [t.stringLiteral(record.type)];
+
+  if (record.type === "break" ||
+      record.type === "continue") {
+    t.assertLiteral(record.target);
+    abruptArgs[1] = record.target;
+  } else if (record.type === "return" ||
+             record.type === "throw") {
+    if (record.value) {
+      t.assertExpression(record.value);
+      abruptArgs[1] = record.value;
+    }
+  }
+
+  this.emit(
+    t.returnStatement(
+      t.callExpression(
+        this.contextProperty("abrupt"),
+        abruptArgs
+      )
+    )
+  );
+};
+
+function isValidCompletion(record) {
+  let type = record.type;
+
+  if (type === "normal") {
+    return !hasOwn.call(record, "target");
+  }
+
+  if (type === "break" ||
+      type === "continue") {
+    return !hasOwn.call(record, "value")
+        && util.getTypes().isLiteral(record.target);
+  }
+
+  if (type === "return" ||
+      type === "throw") {
+    return hasOwn.call(record, "value")
+        && !hasOwn.call(record, "target");
+  }
+
+  return false;
+}
+
+
+// Not all offsets into emitter.listing are potential jump targets. For
+// example, execution typically falls into the beginning of a try block
+// without jumping directly there. This method returns the current offset
+// without marking it, so that a switch case will not necessarily be
+// generated for this offset (I say "not necessarily" because the same
+// location might end up being marked in the process of emitting other
+// statements). There's no logical harm in marking such locations as jump
+// targets, but minimizing the number of switch cases keeps the generated
+// code shorter.
+Ep.getUnmarkedCurrentLoc = function() {
+  return util.getTypes().numericLiteral(this.listing.length);
+};
+
+// The context.prev property takes the value of context.next whenever we
+// evaluate the switch statement discriminant, which is generally good
+// enough for tracking the last location we jumped to, but sometimes
+// context.prev needs to be more precise, such as when we fall
+// successfully out of a try block and into a finally block without
+// jumping. This method exists to update context.prev to the freshest
+// available location. If we were implementing a full interpreter, we
+// would know the location of the current instruction with complete
+// precision at all times, but we don't have that luxury here, as it would
+// be costly and verbose to set context.prev before every statement.
+Ep.updateContextPrevLoc = function(loc) {
+  if (loc) {
+    util.getTypes().assertLiteral(loc);
+
+    if (loc.value === -1) {
+      // If an uninitialized location literal was passed in, set its value
+      // to the current this.listing.length.
+      loc.value = this.listing.length;
+    } else {
+      // Otherwise assert that the location matches the current offset.
+      assert.strictEqual(loc.value, this.listing.length);
+    }
+
+  } else {
+    loc = this.getUnmarkedCurrentLoc();
+  }
+
+  // Make sure context.prev is up to date in case we fell into this try
+  // statement without jumping to it. TODO Consider avoiding this
+  // assignment when we know control must have jumped here.
+  this.emitAssign(this.contextProperty("prev"), loc);
+};
+
+Ep.explodeExpression = function(path, ignoreResult) {
+  const t = util.getTypes();
+  let expr = path.node;
+  if (expr) {
+    t.assertExpression(expr);
+  } else {
+    return expr;
+  }
+
+  let self = this;
+  let result; // Used optionally by several cases below.
+  let after;
+
+  function finish(expr) {
+    t.assertExpression(expr);
+    if (ignoreResult) {
+      self.emit(expr);
+    } else {
+      return expr;
+    }
+  }
+
+  // If the expression does not contain a leap, then we either emit the
+  // expression as a standalone statement or return it whole.
+  if (!meta.containsLeap(expr)) {
+    return finish(expr);
+  }
+
+  // If any child contains a leap (such as a yield or labeled continue or
+  // break statement), then any sibling subexpressions will almost
+  // certainly have to be exploded in order to maintain the order of their
+  // side effects relative to the leaping child(ren).
+  let hasLeapingChildren = meta.containsLeap.onlyChildren(expr);
+
+  // In order to save the rest of explodeExpression from a combinatorial
+  // trainwreck of special cases, explodeViaTempVar is responsible for
+  // deciding when a subexpression needs to be "exploded," which is my
+  // very technical term for emitting the subexpression as an assignment
+  // to a temporary variable and the substituting the temporary variable
+  // for the original subexpression. Think of exploded view diagrams, not
+  // Michael Bay movies. The point of exploding subexpressions is to
+  // control the precise order in which the generated code realizes the
+  // side effects of those subexpressions.
+  function explodeViaTempVar(tempVar, childPath, ignoreChildResult) {
+    assert.ok(
+      !ignoreChildResult || !tempVar,
+      "Ignoring the result of a child expression but forcing it to " +
+        "be assigned to a temporary variable?"
+    );
+
+    let result = self.explodeExpression(childPath, ignoreChildResult);
+
+    if (ignoreChildResult) {
+      // Side effects already emitted above.
+
+    } else if (tempVar || (hasLeapingChildren &&
+                           !t.isLiteral(result))) {
+      // If tempVar was provided, then the result will always be assigned
+      // to it, even if the result does not otherwise need to be assigned
+      // to a temporary variable.  When no tempVar is provided, we have
+      // the flexibility to decide whether a temporary variable is really
+      // necessary.  Unfortunately, in general, a temporary variable is
+      // required whenever any child contains a yield expression, since it
+      // is difficult to prove (at all, let alone efficiently) whether
+      // this result would evaluate to the same value before and after the
+      // yield (see #206).  One narrow case where we can prove it doesn't
+      // matter (and thus we do not need a temporary variable) is when the
+      // result in question is a Literal value.
+      result = self.emitAssign(
+        tempVar || self.makeTempVar(),
+        result
+      );
+    }
+    return result;
+  }
+
+  // If ignoreResult is true, then we must take full responsibility for
+  // emitting the expression with all its side effects, and we should not
+  // return a result.
+
+  switch (expr.type) {
+  case "MemberExpression":
+    return finish(t.memberExpression(
+      self.explodeExpression(path.get("object")),
+      expr.computed
+        ? explodeViaTempVar(null, path.get("property"))
+        : expr.property,
+      expr.computed
+    ));
+
+  case "CallExpression":
+    let calleePath = path.get("callee");
+    let argsPath = path.get("arguments");
+
+    let newCallee;
+    let newArgs = [];
+
+    let hasLeapingArgs = false;
+    argsPath.forEach(function(argPath) {
+      hasLeapingArgs = hasLeapingArgs ||
+        meta.containsLeap(argPath.node);
+    });
+
+    if (t.isMemberExpression(calleePath.node)) {
+      if (hasLeapingArgs) {
+        // If the arguments of the CallExpression contained any yield
+        // expressions, then we need to be sure to evaluate the callee
+        // before evaluating the arguments, but if the callee was a member
+        // expression, then we must be careful that the object of the
+        // member expression still gets bound to `this` for the call.
+
+        let newObject = explodeViaTempVar(
+          // Assign the exploded callee.object expression to a temporary
+          // variable so that we can use it twice without reevaluating it.
+          self.makeTempVar(),
+          calleePath.get("object")
+        );
+
+        let newProperty = calleePath.node.computed
+          ? explodeViaTempVar(null, calleePath.get("property"))
+          : calleePath.node.property;
+
+        newArgs.unshift(newObject);
+
+        newCallee = t.memberExpression(
+          t.memberExpression(
+            newObject,
+            newProperty,
+            calleePath.node.computed
+          ),
+          t.identifier("call"),
+          false
+        );
+
+      } else {
+        newCallee = self.explodeExpression(calleePath);
+      }
+
+    } else {
+      newCallee = explodeViaTempVar(null, calleePath);
+
+      if (t.isMemberExpression(newCallee)) {
+        // If the callee was not previously a MemberExpression, then the
+        // CallExpression was "unqualified," meaning its `this` object
+        // should be the global object. If the exploded expression has
+        // become a MemberExpression (e.g. a context property, probably a
+        // temporary variable), then we need to force it to be unqualified
+        // by using the (0, object.property)(...) trick; otherwise, it
+        // will receive the object of the MemberExpression as its `this`
+        // object.
+        newCallee = t.sequenceExpression([
+          t.numericLiteral(0),
+          newCallee
+        ]);
+      }
+    }
+
+    argsPath.forEach(function(argPath) {
+      newArgs.push(explodeViaTempVar(null, argPath));
+    });
+
+    return finish(t.callExpression(
+      newCallee,
+      newArgs
+    ));
+
+  case "NewExpression":
+    return finish(t.newExpression(
+      explodeViaTempVar(null, path.get("callee")),
+      path.get("arguments").map(function(argPath) {
+        return explodeViaTempVar(null, argPath);
+      })
+    ));
+
+  case "ObjectExpression":
+    return finish(t.objectExpression(
+      path.get("properties").map(function(propPath) {
+        if (propPath.isObjectProperty()) {
+          return t.objectProperty(
+            propPath.node.key,
+            explodeViaTempVar(null, propPath.get("value")),
+            propPath.node.computed
+          );
+        } else {
+          return propPath.node;
+        }
+      })
+    ));
+
+  case "ArrayExpression":
+    return finish(t.arrayExpression(
+      path.get("elements").map(function(elemPath) {
+        return explodeViaTempVar(null, elemPath);
+      })
+    ));
+
+  case "SequenceExpression":
+    let lastIndex = expr.expressions.length - 1;
+
+    path.get("expressions").forEach(function(exprPath) {
+      if (exprPath.key === lastIndex) {
+        result = self.explodeExpression(exprPath, ignoreResult);
+      } else {
+        self.explodeExpression(exprPath, true);
+      }
+    });
+
+    return result;
+
+  case "LogicalExpression":
+    after = loc();
+
+    if (!ignoreResult) {
+      result = self.makeTempVar();
+    }
+
+    let left = explodeViaTempVar(result, path.get("left"));
+
+    if (expr.operator === "&&") {
+      self.jumpIfNot(left, after);
+    } else {
+      assert.strictEqual(expr.operator, "||");
+      self.jumpIf(left, after);
+    }
+
+    explodeViaTempVar(result, path.get("right"), ignoreResult);
+
+    self.mark(after);
+
+    return result;
+
+  case "ConditionalExpression":
+    let elseLoc = loc();
+    after = loc();
+    let test = self.explodeExpression(path.get("test"));
+
+    self.jumpIfNot(test, elseLoc);
+
+    if (!ignoreResult) {
+      result = self.makeTempVar();
+    }
+
+    explodeViaTempVar(result, path.get("consequent"), ignoreResult);
+    self.jump(after);
+
+    self.mark(elseLoc);
+    explodeViaTempVar(result, path.get("alternate"), ignoreResult);
+
+    self.mark(after);
+
+    return result;
+
+  case "UnaryExpression":
+    return finish(t.unaryExpression(
+      expr.operator,
+      // Can't (and don't need to) break up the syntax of the argument.
+      // Think about delete a[b].
+      self.explodeExpression(path.get("argument")),
+      !!expr.prefix
+    ));
+
+  case "BinaryExpression":
+    return finish(t.binaryExpression(
+      expr.operator,
+      explodeViaTempVar(null, path.get("left")),
+      explodeViaTempVar(null, path.get("right"))
+    ));
+
+  case "AssignmentExpression":
+    return finish(t.assignmentExpression(
+      expr.operator,
+      self.explodeExpression(path.get("left")),
+      self.explodeExpression(path.get("right"))
+    ));
+
+  case "UpdateExpression":
+    return finish(t.updateExpression(
+      expr.operator,
+      self.explodeExpression(path.get("argument")),
+      expr.prefix
+    ));
+
+  case "YieldExpression":
+    after = loc();
+    let arg = expr.argument && self.explodeExpression(path.get("argument"));
+
+    if (arg && expr.delegate) {
+      let result = self.makeTempVar();
+
+      let ret = t.returnStatement(t.callExpression(
+        self.contextProperty("delegateYield"),
+        [
+          arg,
+          t.stringLiteral(result.property.name),
+          after
+        ]
+      ));
+      ret.loc = expr.loc;
+
+      self.emit(ret);
+      self.mark(after);
+
+      return result;
+    }
+
+    self.emitAssign(self.contextProperty("next"), after);
+
+    let ret = t.returnStatement(arg || null);
+    // Preserve the `yield` location so that source mappings for the statements
+    // link back to the yield properly.
+    ret.loc = expr.loc;
+    self.emit(ret);
+    self.mark(after);
+
+    return self.contextProperty("sent");
+
+  default:
+    throw new Error(
+      "unknown Expression of type " +
+        JSON.stringify(expr.type));
+  }
+};