先收集一些资料，占个坑。下周末有空写

let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
let n = { x, y, ...z, x:42 };

let [a,b, ...c] = [x, y, z, n];
let d = [a, b, ...[5, 6]];

function f (a,b, ...c) {
  console.log(a,b,c)
}

function f1 ({a,b, ...c}, d) {
  //destructing the first argument
  console.log(a,b,c)
}

"use strict";

var _extends = Object.assign || function(target) {
  for (var i = 1; i < arguments.length; i++) {
    var source = arguments[i];
    for (var key in source) {
      if (Object.prototype.hasOwnProperty.call(source, key)) {
        target[key] = source[key];
      }
    }
  }
  return target;
};

function _defineProperty(obj, key, value) {
  if (key in obj) {
    Object.defineProperty(obj, key, {
      value: value,
      enumerable: true,
      configurable: true,
      writable: true
    });
  } else {
    obj[key] = value;
  }
  return obj;
}

function _objectWithoutProperties(obj, keys) {
  var target = {};
  for (var i in obj) {
    if (keys.indexOf(i) >= 0) continue;
    if (!Object.prototype.hasOwnProperty.call(obj, i)) continue;
    target[i] = obj[i];
  }
  return target;
}

var _x$y$a$b = {
    x: 1,
    y: 2,
    a: 3,
    b: 4
  },
  x = _x$y$a$b.x,
  y = _x$y$a$b.y,
  z = _objectWithoutProperties(_x$y$a$b, ["x", "y"]);

var n = _extends({
  x: x,
  y: y
}, z, _defineProperty({}, "x", 42));

var a = x,
  b = y,
  c = [z, n];

var d = [a, b].concat([5, 6]);

function f(a, b) {
  for (var _len = arguments.length, c = Array(_len > 2 ? _len - 2 : 0), _key = 2; _key < _len; _key++) {
    c[_key - 2] = arguments[_key];
  }

  console.log(a, b, c);
}

function f1(_ref, d) {
  var a = _ref.a,
    b = _ref.b,
    c = _objectWithoutProperties(_ref, ["a", "b"]);

  //destructing the first argument
  console.log(a, b, c);
}

 

 

Array Destructing

 

Array Literal

 

Object Destructing

 

Object Literal

 

The Spec

https://tc39.github.io/proposal-object-rest-spread/

 

V8 Implementation

Github/node/deps/v8/src/interpreter/bytecode-generator.cc

void BytecodeGenerator::BuildArrayLiteralSpread(Spread* spread, Register array,
                                                Register index,
                                                FeedbackSlot index_slot,
                                                FeedbackSlot element_slot) {
  RegisterAllocationScope register_scope(this);
  Register value = register_allocator()->NewRegister();

  builder()->SetExpressionAsStatementPosition(spread->expression());
  IteratorRecord iterator =
      BuildGetIteratorRecord(spread->expression(), IteratorType::kNormal);

  LoopBuilder loop_builder(builder(), nullptr, nullptr);
  loop_builder.LoopHeader();

  // Call the iterator's .next() method. Break from the loop if the `done`
  // property is truthy, otherwise load the value from the iterator result and
  // append the argument.
  BuildIteratorNext(iterator, value);
  builder()->LoadNamedProperty(
      value, ast_string_constants()->done_string(),
      feedback_index(feedback_spec()->AddLoadICSlot()));
  loop_builder.BreakIfTrue(ToBooleanMode::kConvertToBoolean);

  loop_builder.LoopBody();
  builder()
      // value = value.value
      ->LoadNamedProperty(value, ast_string_constants()->value_string(),
                          feedback_index(feedback_spec()->AddLoadICSlot()))
      .StoreAccumulatorInRegister(value)
      // array[index] = value
      .StoreInArrayLiteral(array, index, feedback_index(element_slot))
      // index++
      .LoadAccumulatorWithRegister(index)
      .UnaryOperation(Token::INC, feedback_index(index_slot))
      .StoreAccumulatorInRegister(index);
  loop_builder.BindContinueTarget();
  loop_builder.JumpToHeader(loop_depth_);
}

 

void BytecodeGenerator::VisitObjectLiteral(ObjectLiteral* expr) {
  expr->InitDepthAndFlags();

  // Fast path for the empty object literal which doesn't need an
  // AllocationSite.
  if (expr->IsEmptyObjectLiteral()) {
    DCHECK(expr->IsFastCloningSupported());
    builder()->CreateEmptyObjectLiteral();
    return;
  }

  // Deep-copy the literal boilerplate.
  uint8_t flags = CreateObjectLiteralFlags::Encode(
      expr->ComputeFlags(), expr->IsFastCloningSupported());

  Register literal = register_allocator()->NewRegister();

  // Create literal object.
  int property_index = 0;
  bool clone_object_spread =
      expr->properties()->first()->kind() == ObjectLiteral::Property::SPREAD;
  if (clone_object_spread) {
    // Avoid the slow path for spreads in the following common cases:
    //   1) `let obj = { ...source }`
    //   2) `let obj = { ...source, override: 1 }`
    //   3) `let obj = { ...source, ...overrides }`
    RegisterAllocationScope register_scope(this);
    Expression* property = expr->properties()->first()->value();
    Register from_value = VisitForRegisterValue(property);

    BytecodeLabels clone_object(zone());
    builder()->JumpIfUndefined(clone_object.New());
    builder()->JumpIfNull(clone_object.New());
    builder()->ToObject(from_value);

    clone_object.Bind(builder());
    int clone_index = feedback_index(feedback_spec()->AddCloneObjectSlot());
    builder()->CloneObject(from_value, flags, clone_index);
    builder()->StoreAccumulatorInRegister(literal);
    property_index++;
  } else {
    size_t entry;
    // If constant properties is an empty fixed array, use a cached empty fixed
    // array to ensure it's only added to the constant pool once.
    if (expr->properties_count() == 0) {
      entry = builder()->EmptyObjectBoilerplateDescriptionConstantPoolEntry();
    } else {
      entry = builder()->AllocateDeferredConstantPoolEntry();
      object_literals_.push_back(std::make_pair(expr, entry));
    }
    BuildCreateObjectLiteral(literal, flags, entry);
  }

  // Store computed values into the literal.
  AccessorTable accessor_table(zone());
  for (; property_index < expr->properties()->length(); property_index++) {
    ObjectLiteral::Property* property = expr->properties()->at(property_index);
    if (property->is_computed_name()) break;
    if (!clone_object_spread && property->IsCompileTimeValue()) continue;

    RegisterAllocationScope inner_register_scope(this);
    Literal* key = property->key()->AsLiteral();
    switch (property->kind()) {
      case ObjectLiteral::Property::SPREAD:
        UNREACHABLE();
      case ObjectLiteral::Property::CONSTANT:
      case ObjectLiteral::Property::MATERIALIZED_LITERAL:
        DCHECK(clone_object_spread || !property->value()->IsCompileTimeValue());
        V8_FALLTHROUGH;
      case ObjectLiteral::Property::COMPUTED: {
        // It is safe to use [[Put]] here because the boilerplate already
        // contains computed properties with an uninitialized value.
        if (key->IsStringLiteral()) {
          DCHECK(key->IsPropertyName());
          if (property->emit_store()) {
            builder()->SetExpressionPosition(property->value());
            VisitForAccumulatorValue(property->value());
            FeedbackSlot slot = feedback_spec()->AddStoreOwnICSlot();
            if (FunctionLiteral::NeedsHomeObject(property->value())) {
              RegisterAllocationScope register_scope(this);
              Register value = register_allocator()->NewRegister();
              builder()->StoreAccumulatorInRegister(value);
              builder()->StoreNamedOwnProperty(
                  literal, key->AsRawPropertyName(), feedback_index(slot));
              VisitSetHomeObject(value, literal, property);
            } else {
              builder()->StoreNamedOwnProperty(
                  literal, key->AsRawPropertyName(), feedback_index(slot));
            }
          } else {
            builder()->SetExpressionPosition(property->value());
            VisitForEffect(property->value());
          }
        } else {
          RegisterList args = register_allocator()->NewRegisterList(4);

          builder()->MoveRegister(literal, args[0]);
          builder()->SetExpressionPosition(property->key());
          VisitForRegisterValue(property->key(), args[1]);
          builder()->SetExpressionPosition(property->value());
          VisitForRegisterValue(property->value(), args[2]);
          if (property->emit_store()) {
            builder()
                ->LoadLiteral(Smi::FromEnum(LanguageMode::kSloppy))
                .StoreAccumulatorInRegister(args[3])
                .CallRuntime(Runtime::kSetProperty, args);
            Register value = args[2];
            VisitSetHomeObject(value, literal, property);
          }
        }
        break;
      }
      case ObjectLiteral::Property::PROTOTYPE: {
        // __proto__:null is handled by CreateObjectLiteral.
        if (property->IsNullPrototype()) break;
        DCHECK(property->emit_store());
        DCHECK(!property->NeedsSetFunctionName());
        RegisterList args = register_allocator()->NewRegisterList(2);
        builder()->MoveRegister(literal, args[0]);
        builder()->SetExpressionPosition(property->value());
        VisitForRegisterValue(property->value(), args[1]);
        builder()->CallRuntime(Runtime::kInternalSetPrototype, args);
        break;
      }
      case ObjectLiteral::Property::GETTER:
        if (property->emit_store()) {
          accessor_table.lookup(key)->second->getter = property;
        }
        break;
      case ObjectLiteral::Property::SETTER:
        if (property->emit_store()) {
          accessor_table.lookup(key)->second->setter = property;
        }
        break;
    }
  }

  // Define accessors, using only a single call to the runtime for each pair of
  // corresponding getters and setters.
  for (AccessorTable::Iterator it = accessor_table.begin();
       it != accessor_table.end(); ++it) {
    RegisterAllocationScope inner_register_scope(this);
    RegisterList args = register_allocator()->NewRegisterList(5);
    builder()->MoveRegister(literal, args[0]);
    VisitForRegisterValue(it->first, args[1]);
    VisitObjectLiteralAccessor(literal, it->second->getter, args[2]);
    VisitObjectLiteralAccessor(literal, it->second->setter, args[3]);
    builder()
        ->LoadLiteral(Smi::FromInt(NONE))
        .StoreAccumulatorInRegister(args[4])
        .CallRuntime(Runtime::kDefineAccessorPropertyUnchecked, args);
  }

  // Object literals have two parts. The "static" part on the left contains no
  // computed property names, and so we can compute its map ahead of time; see
  // Runtime_CreateObjectLiteralBoilerplate. The second "dynamic" part starts
  // with the first computed property name and continues with all properties to
  // its right. All the code from above initializes the static component of the
  // object literal, and arranges for the map of the result to reflect the
  // static order in which the keys appear. For the dynamic properties, we
  // compile them into a series of "SetOwnProperty" runtime calls. This will
  // preserve insertion order.
  for (; property_index < expr->properties()->length(); property_index++) {
    ObjectLiteral::Property* property = expr->properties()->at(property_index);
    RegisterAllocationScope inner_register_scope(this);

    if (property->IsPrototype()) {
      // __proto__:null is handled by CreateObjectLiteral.
      if (property->IsNullPrototype()) continue;
      DCHECK(property->emit_store());
      DCHECK(!property->NeedsSetFunctionName());
      RegisterList args = register_allocator()->NewRegisterList(2);
      builder()->MoveRegister(literal, args[0]);
      builder()->SetExpressionPosition(property->value());
      VisitForRegisterValue(property->value(), args[1]);
      builder()->CallRuntime(Runtime::kInternalSetPrototype, args);
      continue;
    }

    switch (property->kind()) {
      case ObjectLiteral::Property::CONSTANT:
      case ObjectLiteral::Property::COMPUTED:
      case ObjectLiteral::Property::MATERIALIZED_LITERAL: {
        Register key = register_allocator()->NewRegister();
        BuildLoadPropertyKey(property, key);
        builder()->SetExpressionPosition(property->value());
        Register value = VisitForRegisterValue(property->value());
        VisitSetHomeObject(value, literal, property);

        DataPropertyInLiteralFlags data_property_flags =
            DataPropertyInLiteralFlag::kNoFlags;
        if (property->NeedsSetFunctionName()) {
          data_property_flags |= DataPropertyInLiteralFlag::kSetFunctionName;
        }

        FeedbackSlot slot =
            feedback_spec()->AddStoreDataPropertyInLiteralICSlot();
        builder()
            ->LoadAccumulatorWithRegister(value)
            .StoreDataPropertyInLiteral(literal, key, data_property_flags,
                                        feedback_index(slot));
        break;
      }
      case ObjectLiteral::Property::GETTER:
      case ObjectLiteral::Property::SETTER: {
        RegisterList args = register_allocator()->NewRegisterList(4);
        builder()->MoveRegister(literal, args[0]);
        BuildLoadPropertyKey(property, args[1]);
        builder()->SetExpressionPosition(property->value());
        VisitForRegisterValue(property->value(), args[2]);
        VisitSetHomeObject(args[2], literal, property);
        builder()
            ->LoadLiteral(Smi::FromInt(NONE))
            .StoreAccumulatorInRegister(args[3]);
        Runtime::FunctionId function_id =
            property->kind() == ObjectLiteral::Property::GETTER
                ? Runtime::kDefineGetterPropertyUnchecked
                : Runtime::kDefineSetterPropertyUnchecked;
        builder()->CallRuntime(function_id, args);
        break;
      }
      case ObjectLiteral::Property::SPREAD: {
        RegisterList args = register_allocator()->NewRegisterList(2);
        builder()->MoveRegister(literal, args[0]);
        builder()->SetExpressionPosition(property->value());
        VisitForRegisterValue(property->value(), args[1]);
        builder()->CallRuntime(Runtime::kCopyDataProperties, args);
        break;
      }
      case ObjectLiteral::Property::PROTOTYPE:
        UNREACHABLE();  // Handled specially above.
        break;
    }
  }

  builder()->LoadAccumulatorWithRegister(literal);
}

void BytecodeGenerator::VisitObjectLiteral(ObjectLiteral* expr) {
  // 针对以下三种常用的spread模式进行优化
    // 1) `let obj = { ...source }`
    // 2) `let obj = { ...source, override: 1 }`
    // 3) `let obj = { ...source, ...overrides }`
  // 静态部分：属性名是字符串
  // 动态部分：属性名需要计算
    // ObjectLiteral::Property::SPREAD 调用运行时 CopyDataProperties
}

 

References：

• Understanding V8’s Bytecode