Somehow it looks like reflect in Java.

For example: We define an mothod on the Object, it called defineMethod(). It accepts two arguements, one is methodName andother is methodBody. 

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Using defineProperty() method of Object object to create method.

Object.prototype.defineMethod = function(methodName, methodBody){    Object.defineProperty(this, methodName, {    enumerable: true,    configurable: true,    value: methodBody  });}var dog = {breed: "Shelty"};dog.defineMethod("bark", function(){  return "Woof!";});console.log(dog.breed);console.log(dog.bark());//"Shelty"//"Woof!"

 

More useful case:

function User(){    User.statuses = ["inactive", "active"];  _.each(User.statuses, function(status){    this.defineMethod("is"+status.capitalize(), function(){      return this.status == status;    })  }, this);}var user = new User();user.status = "active";console.log(user.isActive());console.log(user.isInactive());//isActive() and isInactive() methods are created dynamcally during the running time.

Library: lodash  and active-support.js

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