Objects

Question 1

Object.length

Answer 1

Has a value of 1

Question 2

Object.prototype

Answer 2

• Allows addition of properties to all objects of type Object

Question 3

Object.assign()

Answer 3

^{Object Constructor Methods}

Object.assign(target, ...sources)

Description

• Used to copy the values of all enumerable own properties from one or more source objects to a target object.
• Properties in the target object will be overwritten by properties in the sources if they have the same key. Later sources’ properties will similarly overwrite earlier ones.

Parameters

• target - the target object
• sources - the source object(s)

Notes

• Uses [[Get]] on the sources and [[Set]] on the target, so it will invoke getters and setters.
• Assigns properties versus just copying or defining new properties (aka the entire definition including getters). For copying property definitions, including enumerability, into prototypes, use Object.getOwnPropertyDescriptor() and Object.defineProperties().
• Both String and Symbol properties are copied
• Primitives are wrapped to object
  
  • Only strings are enumerable though.
• Deep clone an object: (use the following in an Object.assign source) JSON.parse(JSON.stringify(obj))
• If a property on the target is read-only, and a source tries to overwrite it, TypeError will be thrown
• Exceptions will interrupt the ongoing copying task (not atomic transactions)
• To copy whole descriptor, not just simple assignment, must go through object’s keys and symbols and use getOwnPropertyDescriptor

Thoughts

• Use Object.assign(this, {x, y}) in constructor to quickly setup instance properties
• Instead of MyClass.prototype.foo, can use assign() instead:

Object.assign(MyClass.prototype, {
` foo(arg1, arg2) { //… } }`

• Shallow copy (including prototype):
  
  • let newObj = Object.assign({\_\_proto\_\_: obj.\_\_proto\_\_}, obj);
  • Major performance pitfalls using this
  • Use Object.create() to get desired [[Prototype]] instead
• If shallow copying only own properties:
  
  • let newObj = Object.assign({}, obj);

Question 4

Object.create()

Answer 4

^{Object Constructor Methods}

Object.create(proto[, propertiesObject])

Description

• Creates a new object with the specified prototype and properties

Parameters

• proto - The object which should be the prototype of the newly-created object
• propertiesObject - property descriptors to be added to the newly-created object, with the corresponding property names. These properties correspond to the second argument of Object.defineProperties()

Return

• A new object with the specified prototype and properties

Exceptions

• TypeError thrown if propertiesObject isn’t null or an object

Notes

• Classical inheritance using functions as classes

Rectangle.prototype = Object.create(Shape.prototype);Rectangle.prototype.constructor = Rectangle;

• Can achieve quasi-multiple-inheritance using mixins

MyClass.prototype = Object.create(SuperClass.prototype);Object.assign(MyClass.prototype, OtherSuperClass.prototype);MyClass.prototype.constructor = MyClass;

• o = {} equivalent to o = Object.create(Object.prototype)
• By default properties are not writable, enumerable, or configurable