Objects

Question 1

3 ways to create a blank object

Answer 1

{}
new Object
Object.create(null)

Question 2

Count number of enumerable properties an object has

Answer 2

Object.keys( obj ).length

Question 3

Test if object contains property/method, whether directly or in the prototype chain.

Answer 3

“field_name” in obj

Do not use obj.field_name, as falsey values will evaluate to false. in also has the added benefit of the properties not being evaluated, which could be negative on performance.

Question 4

Test if object contains property/method directly (owns).

Answer 4

obj.hasOwnProperty(“field_name”);

Question 5

How to remove a property/method from an object?

Answer 5

delete obj.field_name //=> true|false

(properties with “configurable” set to false cannot be removed, so false would be returned. false is also returned if the property doesn’t exist)

Question 6

Dereferencing an object?

Answer 6

Dereferencing an object tells JS that you no longer need the object so the built-in garbage collector can free up the memory being used by that object. This happens automatically in JS, but you need to dereference the object first; the best way to do that is set the object pointer’s variable to null:

var o = {};
// do something
o = null;  // if there aren't any other pointers stored to a variable, JS will garbage collect it :)

Question 7

3 ways to define a property (data-property)

Answer 7

(property fields must always be strings; if not observed, it is converted into a string)

o["prop"] = 1;
o.prop = 1;    // if the field name *could* be a valid variable name

// ECMAScript 5 way
// (3rd argument is called "Property Descriptor")
Object.defineProperty(o, "prop", {
    value: 1,
    // if a value can be reassigned
    writable: true,
    enumerable: true,
    // if prop can be deleted
    // and if you can modify "property descriptor"
    configurable: true,
})

Question 8

Make a child object of a parent object

Answer 8

var parent = {};

…

var child = Object.create(parent);

Question 9

2 ways to access a property.

Answer 9

var o = { name: “Julie”, greet: function () { } };

o. name
o. greet()

o[“name”]
o“greet”

The latter is useful for situations like:
var meth = “greet”;
ometh;

Question 10

Test if property is enumerable

Answer 10

Object.prototype.propertyIsEnumerable(“field-name”);

[].propertyIsEnumberable(“length”) //=> false

Question 11

Two types of properties.

Answer 11

Data properties & accessor properties.

Data properties contain a value. Accessor properties contain a getter and/or setter function.

Question 12

2 ways to create an accessor property

Answer 12

Literal Syntax:
{
  get getterProp() {
    return "J";
  },
  set setterProp(value) {
    console.log(value);
  }
}

ES5 Method:
Object.defineProperty(obj, "propName", {
  enumerable: true,
  configurable: true,
  get: function () { return "J"; },
  set: function (value) { console.log(value); }
})

Question 13

Convention to indicate that property should be treated like it’s private (even if it’s still public)

Answer 13

Prepend with underscore

_privProp

Question 14

What 2 attributes do data & accessor properties share in common?

Answer 14

enumerable & configurable

Question 15

What are the default attributes if you use the literal property assignment method: obj.name = "value"

Answer 15

enumerable = true
configurable = true
writable = true

Question 16

What are the default property descriptor attributes if you use the ES5 Object.defineProperty method?

What about if the property already exists?

Answer 16

Every attribute not included in your property descriptor object will default to false.

However, if the property exists, only values included in the descriptor object will change the attribute values.

Question 17

Add/modify multiple properties to object using ES5 method

Answer 17

Object.defineProperties(obj, {
propName : { /* prop-descriptor */ },
…
});

Question 18

Get property’s attributes

Answer 18

Object.getOwnPropertyDescriptor(obj, "field-name")
// if data property => { enumerable, configurable, writable, value }
// if accessor property => { enumerable, configurable, set, get }

Question 19

How to prevent an object from receiving new attributes?

How to check if this occurred on an object?

Answer 19

Object.preventExtensions( obj )

// check if object is extensible
Object.isExtensible( obj )  //=> true|false

Question 20

What is a “sealed” object?

How to check if this occurred?

Answer 20

A “sealed” object means that you can only read or write to the object’s properties, you cannot add new properties and all existing properties become non-configurable.

Object.seal( obj )

// check if object is sealed
Object.isSealed( obj )

Question 21

What is a frozen object?

How to check if this occurred?

Answer 21

A “frozen” object is a sealed object + you cannot write to any properties. In essence, it’s a read only object. Frozen objects can not become unfrozen.

Object.freeze( obj )

// check if object is frozen
Object.isFrozen( obj )