在JavaScript中模拟’new’运算符

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我尝试用这样的代码在JavaScript中模拟’new’运算符：

Function.method('new', function ( ) {
    var objPrototype = Object.create(this.prototype);
    var instance = this.apply(objPrototype, arguments);

    return instance;
});

但是,为了涵盖所有情况,return语句应如下所示：

return (typeof instance === 'object' && instance ) || objPrototype;

现在进行测试：

var SomeClass = function (param1, param2) {
    this.param1 = param1;
    this.param2 = param2;
};

var test1 = String.new('test1'); //in this case, the "instance" variable is an object
var test2 = SomeClass.new('test1', 'test2'); // in this case, the "instance" variable is undefined

这正是“新”运算符的作用吗？还有什么案子可以解决吗？

解决方法:

从specification：

11.2.2 The new Operator 07001 07002 07003 07004

The production NewExpression : new NewExpression is evaluated as follows:

1. Let ref be the result of evaluating NewExpression.
2. Let constructor be 07005(ref).
3. If 07006(constructor) is not Object, throw a 07007 exception.
4. If constructor does not implement the [[Construct]] internal method, throw a 07007 exception.
5. Return the result of calling the [[Construct]] internal method on constructor, providing no arguments (that is, an empty list of arguments).

The production MemberExpression : new MemberExpression Arguments
is evaluated as follows:

1. Let ref be the result of evaluating MemberExpression.
2. Let constructor be 07005(ref).
3. Let argList be the result of evaluating Arguments, producing an internal list of argument values (070010).
4. If 07006(constructor) is not Object, throw a 07007 exception.
5. If constructor does not implement the [[Construct]] internal method, throw a 07007 exception.
6. Return the result of calling the [[Construct]] internal method on constructor, providing the list argList as the argument values.

在任何一种情况下,都正确遵循所有步骤：

var objPrototype = Object.create(this.prototype);    // 1-4 1-5
var instance = this.apply(objPrototype, arguments);  // 5   6

兴趣点是2.
The specification for [[construct]]指出：

When the [[Construct]] internal method for a Function object F is
called with a possibly empty list of arguments, the following steps
are taken:

• Let obj be a newly created native ECMAScript object.
  . . .
• Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing the argument list
  passed into [[Construct]] as args.
• If 07006(result) is Object then return result.
• Return obj.

typeof obj为null返回“object”,而null不是对象.但是,由于null是一个假值,您的代码也可以按预期工作：

return (typeof instance === 'object' && instance ) || objPrototype;

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