学步园

转自：http://stackoverflow.com/questions/6750880/javascript-how-does-new-work-internally

The new operator
uses the internal [[Construct]] method,
and it basically does the following:

• Initializes a new native object
• Sets the internal [[Prototype]] of
  this object, pointing to the Function prototype property.
  • If the function's prototype property
    is not an object (a primitive values, such as a Number, String, Boolean, Undefined or Null), Object.prototype is
    used instead.
• After creating the object, it calls the function, providing the object as its this value.
• If the return value of the called function, is a primitive, the object created internally is returned.
• Otherwise, if an object is returned, the object created internally is lost.

An equivalent implementation of what the new operator
does, can be expressed like this (assuming that the ECMAScript 5 Object.create method
is available):