I apologize in advance if this has been asked elsewhere or if I have missed something important in the docs, well, but I need to ask this:
Let's say an object is being created without defining the respective properties
cons objA = Object.create({
init(text) {
this.text = text;
}
});
or an object is being created and the respective property has been declared in the properties object
const objB = Object.create({
init(text) {
this.text = text;
}
}, {
text: {
value: '',
writable: true
}
});
I understand that defining properties in the propertiesObject of Object.create helps defining and providing better contracts,
but
do these two scenarios vary in respect to the text property?
Yes, they vary - there is difference between a and b results
const a = Object.create({
init(text) {
this.text = text;
}
});
const b = Object.create({
init(text) {
this.text = text;
}
}, {
text: {
value: '',
writable: true
}
});
console.log(Object.getOwnPropertyNames(a))
console.log(Object.getOwnPropertyNames(b))
a.init('a text');
b.init('b text');
console.log(Object.getOwnPropertyDescriptor(a, 'text'))
console.log(Object.getOwnPropertyDescriptor(b, 'text'))
a returns:
{
"value": "a text",
"writable": true,
"enumerable": true,
"configurable": true
}
b returns:
{
"value": "b text",
"writable": true,
"enumerable": false,
"configurable": false
}
By default property is enumerable and configurable unless you define property by Object.defineProperty or Object.create. You can define them as you did with writable.
Enumerable works in certain object-property enumerations, such as the for..in.
Configurable means that you cannot use Object.defineProperty again on this property (there will be Cannot redefine property: text error).
Other than that - before you use init() - a object will have undefined text property (that is not enumerable, not configurable, writable) and object b will not have any property.
Related
I was playing with below javascript code. Understanding of Object.defineProperty() and I am facing a strange issue with it. When I try to execute below code in the browser or in the VS code the output is not as expected whereas if I try to debug the code the output is correct
When I debug the code and evaluate the profile I can see the name & age property in the object
But at the time of output, it only shows the name property
//Code Snippet
let profile = {
name: 'Barry Allen',
}
// I added a new property in the profile object.
Object.defineProperty(profile, 'age', {
value: 23,
writable: true
})
console.log(profile)
console.log(profile.age)
Now expected output here should be
{name: "Barry Allen", age: 23}
23
but I get the output as.
Note that I am able to access the age property defined afterwards.
I am not sure why the console.log() is behaving this way.
{name: "Barry Allen"}
23
You should set enumerable to true. In Object.defineProperty its false by default. According to MDN.
enumerable
true if and only if this property shows up during enumeration of the properties on the corresponding object.
Defaults to false.
Non-enumerable means that property will not be shown in Object.keys() or for..in loop neither in console
let profile = {
name: 'Barry Allen',
}
// I added a new property in the profile object.
Object.defineProperty(profile , 'age', {
value: 23,
writable: true,
enumerable: true
})
console.log(profile)
console.log(profile.age)
All the properties and methods on prototype object of built-in classes are non-enumerable. Thats is the reason you can call them from instance but they don't appear while iterating.
To get all properties(including non-enumerable)Object.getOwnPropertyNames()
.
let profile = {
name: 'Barry Allen',
}
// I added a new property in the profile object.
Object.defineProperty(profile , 'age', {
value: 23,
writable: true,
enumerable: false
})
for(let key in profile) console.log(key) //only name will be displayed.
console.log(Object.getOwnPropertyNames(profile)) //You will se age too
By default, properties you define with defineProperty are not enumerable - this means that they will not show up when you iterate over their Object.keys (which is what the snippet console does). (Similarly, the length property of an array does not get displayed, because it's non-enumerable.)
See MDN:
enumerable
true if and only if this property shows up during enumeration of the properties on the corresponding object.
Defaults to false.
Make it enumerable instead:
//Code Snippet
let profile = {
name: 'Barry Allen',
}
// I added a new property in the profile object.
Object.defineProperty(profile, 'age', {
value: 23,
writable: true,
enumerable: true
})
console.log(profile)
console.log(profile.age)
The reason you can see the property in the logged image is that Chrome's console will show you non-enumerable properties as well - but the non-enumerable properties will be slightly greyed-out:
See how age is grey-ish, while name is not - this indicates that name is enumerable, and age is not.
Whenever you use".defineProperty" method of object. You should better define all the properties of the descriptor. Because if you don't define other property descriptor then it assumes default values for all of them which is false. So your console.log checks for all the enumerable : true properties and logs them.
//Code Snippet
let profile = {
name: 'Barry Allen',
}
// I added a new property in the profile object.
Object.defineProperty(profile, 'age', {
value: 23,
writable: true,
enumerable : true,
configurable : true
})
console.log(profile)
console.log(profile.age)
I'm fiddling around with a library called bcoin for node. Running the following code:
chain.on('block', function(block) {
console.log('Connected block to blockchain:');
block.txs.forEach(function(t) {
t.inputs.forEach(function(i) {
console.log(typeof i, i);
console.log(JSON.stringify(i));
});
});
});
This is the response I'm getting:
Connected block to blockchain:
object { type: 'coinbase',
subtype: null,
address: null,
script: <Script: 486604799 676>,
witness: <Witness: >,
redeem: null,
sequence: 4294967295,
prevout: <Outpoint: 0000000000000000000000000000000000000000000000000000000000000000/4294967295>,
coin: null }
{"prevout":{"hash":"0000000000000000000000000000000000000000000000000000000000000000","index":4294967295},"script":"04ffff001d02a402","witness":"00","sequence":4294967295,"address":null}
Notice that even though the attribute type for example, is shown when we print i, that attribute does not exist when we JSON.stringify the object. If I tried to console.log(i.type) I'd get undefined.
How is that possible? And what is a good way of debugging what's going on with an object?
JSON.stringify will only includes enumerable properties that are not functions.
So if you define a property and set as non-enumerable, it will not be a part of JSON string.
var obj = {
a: 'test'
};
// Non-enumerable property
Object.defineProperty(obj, 'type', {
enumerable: false,
value: 'Test'
});
// Get property
Object.defineProperty(obj, 'type2', {
get: function(){
return 'Test 2'
}
});
console.log(JSON.stringify(obj), obj);
console.log(obj.type, obj.type2)
I have been following javascript design patterns and was implementing ES5 properties using the code found at this location.
https://addyosmani.com/resources/essentialjsdesignpatterns/book/#constructorpatternjavascript
Test 1
Use a function to define 3 properties
Check the values are ok: SUCCESS
Log the object to see if the data looks ok: SUCCESS
Test 2
Use Object.definedProperties to define 3 properties
Check the values are ok: SUCCESS
Log the object to see if the data looks ok: FAIL
Test 1 Code
it('should construct object with some single properites added using helper function', function (done) {
var person = Object.create(Object.prototype);
// Populate the object with properties
pattern.defineProp(person, "car", "Delorean");
pattern.defineProp(person, "dateOfBirth", "1981");
pattern.defineProp(person, "hasBeard", false);
console.log(person.car);
console.log(person);
console.log(typeof person);
console.log(JSON.stringify(person));
person.car.should.equal('Delorean');
person.dateOfBirth.should.equal('1981');
person.hasBeard.should.equal(false);
// Produces
// Delorean
// { car: 'Delorean', dateOfBirth: '1981', hasBeard: false }
// object
// {"car":"Delorean","dateOfBirth":"1981","hasBeard":false}
done();
});
Test 2 Code
it('should construct object with multiple properties using standard ES5 syntax', function (done) {
var person = Object.create(Object.prototype);
// Populate the object with properties
Object.defineProperties(person, {
"car": {
value: "Delorean",
writable: true
},
"dateOfBirth": {
value: "1981",
writable: true
},
"hasBeard": {
value: false,
writable: true
}
});
console.log(person.car);
console.log(person);
console.log(typeof person);
console.log(JSON.stringify(person));
person.car.should.equal('Delorean');
person.dateOfBirth.should.equal('1981');
person.hasBeard.should.equal(false);
// Produces
// Delorean
// {}
// object
// {}
done();
});
Define Prop Function
exports.defineProp = function ( obj, key, value ){
var config = {
value: value,
writable: true,
enumerable: true,
configurable: true
};
Object.defineProperty( obj, key, config );
};
Well, if you are using the standard Object.defineProperties, and omit the enumerable attribute, it will default to false. Your properties won't be displayed because they are not enumerable.
Im trying to learn about object.create and prototypal inheritance and have the following:
var Employee = {
'attributes': {},
getAttributes: function() {
return this.attributes;
},
addAttribute: function(attribute) {
if (! this.attributes.hasOwnProperty(attribute)) {
this.attributes.extend(attribute);
}
}
};
var OfficeEmployee = Object.create(Employee);
var OfficeEmployeeInstance = Object.create(OfficeEmployee, {'attributes': {'id': 123, 'name': 'Bob'}});
console.log(OfficeEmployeeInstance.attributes);
OfficeEmployeeInstance.addAttribute({'salary': '100'});
console.log(OfficeEmployeeInstance.getAttributes());
It doesnt work as i expect it should though and throws errors:
console.log(OfficeEmployeeInstance.attributes);
is undefined
and
console.log(OfficeEmployeeInstance.getAttributes());
gives error:
Uncaught TypeError: Cannot call method 'hasOwnProperty' of undefined tester.js:39
Employee.addAttribute tester.js:39
(anonymous function)
What am i doing wrong here?
The second argument of Object.create has to be a properties object. That's an object with a defined structure and specific properties:
var OfficeEmployeeInstance = Object.create(OfficeEmployee, {
'attributes': {
value: {'id': 123, 'name': 'Bob'},
writeable: true,
enumerable: true
}
});
You can find the supported properties here.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create
When using .create you should pass it an argument of someObject.prototype, rather than a constructor name. The documentation above should help.
I am trying to use javascript inheritance in my code. Following is the jsfiddle for the same.
http://jsfiddle.net/Fetsz/3/
var baseObject = Object.create(Object, {
// this works as an instance variable for derived objects
name: {
writable: true,
configurable: true,
enumerable: true,
value: null
},
// this should act as instance variable for derived objects
values: {
writable: true,
configurable: true,
enumerable: true,
value: []
},
displayAlert: {
value: function (){
alert("Alert from base object");
}
}
});
var derivedObj1 = Object.create(baseObject, {});
var derivedObj2 = Object.create(baseObject, {});
function displayValues (obj) {
alert("values for " + obj.name + "\n" + JSON.stringify(obj.values));
};
$(document).ready(function(){
derivedObj1.name = "Object 1";
derivedObj2.name = "Object 2";
derivedObj1.values.push("DO1 element");
derivedObj2.values.push("DO2 element");
derivedObj1.displayAlert();
// should display values added only for derivedObj1
displayValues(derivedObj1);
// should display values added only for derivedObj2
displayValues(derivedObj2);
});
I also checked following question which nicely explains why my code doesn't work. Javascript Inheritance: Parent's array variable retains value
I need to have an array member variable in every derived class which will contain certain values. I was trying to achieve the same by adding array in base class. As mentioned in above question, to achieve the same, I will have to add array member to each derived class. Is there any other way to achieve this without modifying every derived class?
When they are created, both derivedObj1.values and derivedObj2.values point to the same array object. If you were to first reassign each one using derivedObj1.values = [];, in the same way as you are reassigning the name property, your code would behave as expected.
Instead of adding the values property to the original baseObject, you could create an init method that adds an array to the instance:
var baseObject = Object.create(Object, {
// this works as an instance variable for derived objects
name: {
writable: true,
configurable: true,
enumerable: true,
value: null
},
// this should act as instance variable for derived objects
init: {
writable: true,
configurable: true,
enumerable: true,
value: function(){
this.values = [];
}
},
displayAlert: {
value: function (){
alert("Alert from base object");
}
}
});
var derivedObj1 = Object.create(baseObject, {});
derivedObj1.init();
var derivedObj2 = Object.create(baseObject, {});
derivedObj2.init();
Your variables can't be on the prototype if you don't want them to be shared by all instances. You could create a second object with just the property definition for the array,and pass it as the second parameter to Object.create:
var arrDef = {
values: {
writable: true,
configurable: true,
enumerable: true,
value: []
}
}
var derivedObj1 = Object.create(baseObject, arrDef);
var derivedObj2 = Object.create(baseObject, arrDef);