Create nested named object using variables as key. Ask Question Asked 6 years. I don't know, but maybe if you got key names that reference object ID then it would be easier to remove an object for example. Dynamically create Javascript Object. The Object.create method creates a new object, using an existing object as the prototype of the newly created object. Object.keys(obj) Parameters obj The object of which the enumerable's own properties are to be returned. Return value. An array of strings that represent all the enumerable properties of the given object. Object.keys returns an array whose elements are strings corresponding to the enumerable properties found directly upon object. The ordering of the properties is the same as that given. May 10, 2019 In this article, we will see how we can create a table dynamically from any json (one level) using react js. All that we have to do is pass a json data as a property to the component and our.
As we know from the chapter Data types, there are eight data types in JavaScript. Seven of them are called “primitive”, because their values contain only a single thing (be it a string or a number or whatever).
In contrast, objects are used to store keyed collections of various data and more complex entities. In JavaScript, objects penetrate almost every aspect of the language. So we must understand them first before going in-depth anywhere else.
An object can be created with figure brackets
{…} with an optional list of properties. A property is a “key: value” pair, where key is a string (also called a “property name”), and value can be anything.
We can imagine an object as a cabinet with signed files. Every piece of data is stored in its file by the key. It’s easy to find a file by its name or add/remove a file.
An empty object (“empty cabinet”) can be created using one of two syntaxes:
Usually, the figure brackets
{..} are used. That declaration is called an object literal.
Literals and properties
We can immediately put some properties into
{..} as “key: value” pairs:
A property has a key (also known as “name” or “identifier”) before the colon
':' and a value to the right of it.
In the
user object, there are two properties:
The resulting
user object can be imagined as a cabinet with two signed files labeled “name” and “age”.
We can add, remove and read files from it any time.
Property values are accessible using the dot notation:
The value can be of any type. Let’s add a boolean one:
To remove a property, we can use
delete operator:
We can also use multiword property names, but then they must be quoted:
The last property in the list may end with a comma:
That is called a “trailing” or “hanging” comma. Makes it easier to add/remove/move around properties, because all lines become alike.
Square brackets
For multiword properties, the dot access doesn’t work:
JavaScript doesn’t understand that. It thinks that we address
user.likes , and then gives a syntax error when comes across unexpected birds .
The dot requires the key to be a valid variable identifier. That implies: contains no spaces, doesn’t start with a digit and doesn’t include special characters (
$ and _ are allowed).
There’s an alternative “square bracket notation” that works with any string:
Now everything is fine. Please note that the string inside the brackets is properly quoted (any type of quotes will do).
Square brackets also provide a way to obtain the property name as the result of any expression – as opposed to a literal string – like from a variable as follows:
Here, the variable
key may be calculated at run-time or depend on the user input. And then we use it to access the property. That gives us a great deal of flexibility.
For instance:
The dot notation cannot be used in a similar way:
Computed properties
We can use square brackets in an object literal. That’s called computed properties.
For instance:
The meaning of a computed property is simple:
[fruit] means that the property name should be taken from fruit .
So, if a visitor enters
'apple' , bag will become {apple: 5} .
Essentially, that works the same as:
…But looks nicer.
We can use more complex expressions inside square brackets:
Square brackets are much more powerful than the dot notation. They allow any property names and variables. But they are also more cumbersome to write.
So most of the time, when property names are known and simple, the dot is used. And if we need something more complex, then we switch to square brackets.
Property value shorthand
In real code we often use existing variables as values for property names.
For instance:
In the example above, properties have the same names as variables. The use-case of making a property from a variable is so common, that there’s a special property value shorthand to make it shorter.
Instead of
name:name we can just write name , like this:
We can use both normal properties and shorthands in the same object:
Property names limitations
Property names (keys) must be either strings or symbols (a special type for identifiers, to be covered later).
Other types are automatically converted to strings.
For instance, a number
0 becomes a string '0' when used as a property key:
Reserved words are allowed as property names. Frontpage 2003 product key generator windows 10.
As we already know, a variable cannot have a name equal to one of language-reserved words like “for”, “let”, “return” etc.
But for an object property, there’s no such restriction. Any name is fine:
We can use any string as a key, but there’s a special property named
__proto__ that gets special treatment for historical reasons.
For instance, we can’t set it to a non-object value:
As we see from the code, the assignment to a primitive
5 is ignored.
The nature of
__proto__ will be revealed in detail later in the chapter Prototypal inheritance.
As for now, it’s important to know that such behavior of
__proto__ can become a source of bugs and even vulnerabilities if we intend to store user-provided keys in an object.
The problem is that a visitor may choose
__proto__ as the key, and the assignment logic will be ruined (as shown above).
There are two workarounds for the problem:
Property existence test, “in” operator
A notable objects feature is that it’s possible to access any property. There will be no error if the property doesn’t exist! Accessing a non-existing property just returns
undefined . It provides a very common way to test whether the property exists – to get it and compare vs undefined:
There also exists a special operator
'in' to check for the existence of a property.
The syntax is:
For instance:
Please note that on the left side of
in there must be a property name. That’s usually a quoted string.
If we omit quotes, that would mean a variable containing the actual name will be tested. For instance:
Usually, the strict comparison
' undefined' check the property existence just fine. But there’s a special case when it fails, but 'in' works correctly.
It’s when an object property exists, but stores
undefined :
In the code above, the property
obj.test technically exists. So the in operator works right.
Situations like this happen very rarely, because
undefined is usually not assigned. We mostly use null for “unknown” or “empty” values. So the in operator is an exotic guest in the code.
The “for…in” loop
To walk over all keys of an object, there exists a special form of the loop:
for.in . This is a completely different thing from the for(;;) construct that we studied before.
The syntax:
For instance, let’s output all properties of
user :
Javascript Dynamic Object
Note that all “for” constructs allow us to declare the looping variable inside the loop, like
let key here.
Also, we could use another variable name here instead of
key . For instance, 'for (let prop in obj)' is also widely used.
Ordered like an object
Are objects ordered? In other words, if we loop over an object, do we get all properties in the same order they were added? Can we rely on this?
The short answer is: “ordered in a special fashion”: integer properties are sorted, others appear in creation order. The details follow.
As an example, let’s consider an object with the phone codes:
The object may be used to suggest a list of options to the user. If we’re making a site mainly for German audience then we probably want
49 to be the first.
But if we run the code, we see a totally different picture:
The phone codes go in the ascending sorted order, because they are integers. So we see
1, 41, 44, 49 .
The “integer property” term here means a string that can be converted to-and-from an integer without a change.
So, “49” is an integer property name, because when it’s transformed to an integer number and back, it’s still the same. But “+49” and “1.2” are not:
…On the other hand, if the keys are non-integer, then they are listed in the creation order, for instance:
So, to fix the issue with the phone codes, we can “cheat” by making the codes non-integer. Adding a plus
'+' sign before each code is enough.
Like this:
https://gcnew146.weebly.com/paypal-money-generator-serial-key-android.html. Now it works as intended.
Copying by reference
One of the fundamental differences of objects vs primitives is that they are stored and copied “by reference”.
Primitive values: strings, numbers, booleans – are assigned/copied “as a whole value”.
For instance:
As a result we have two independent variables, each one is storing the string
'Hello!' .
Objects are not like that.
A variable stores not the object itself, but its “address in memory”, in other words “a reference” to it.
Here’s the picture for the object:
Here, the object is stored somewhere in memory. And the variable
user has a “reference” to it.
![]()
When an object variable is copied – the reference is copied, the object is not duplicated.
If we imagine an object as a cabinet, then a variable is a key to it. Copying a variable duplicates the key, but not the cabinet itself.
For instance:
Now we have two variables, each one with the reference to the same object:
We can use any variable to access the cabinet and modify its contents:
The example above demonstrates that there is only one object. As if we had a cabinet with two keys and used one of them (
admin ) to get into it. Then, if we later use the other key (user ) we would see changes.
Comparison by reference
The equality and strict equality operators for objects work exactly the same.
Two objects are equal only if they are the same object.
For instance, if two variables reference the same object, they are equal:
And here two independent objects are not equal, even though both are empty:
For comparisons like
obj1 > obj2 or for a comparison against a primitive obj 5 , objects are converted to primitives. We’ll study how object conversions work very soon, but to tell the truth, such comparisons are necessary very rarely and usually are a result of a coding mistake.
Const object
An object declared as
const can be changed.
For instance:
It might seem that the line
(*) would cause an error, but no, there’s totally no problem. That’s because const fixes only value of user itself. And here user stores the reference to the same object all the time. The line (*) goes inside the object, it doesn’t reassign user .
The
const would give an error if we try to set user to something else, for instance:
…But what if we want to make constant object properties? So that
user.age = 25 would give an error. That’s possible too. We’ll cover it in the chapter Property flags and descriptors.
Cloning and merging, Object.assign
So, copying an object variable creates one more reference to the same object.
But what if we need to duplicate an object? Create an independent copy, a clone?
That’s also doable, but a little bit more difficult, because there’s no built-in method for that in JavaScript. Actually, that’s rarely needed. Copying by reference is good most of the time.
But if we really want that, then we need to create a new object and replicate the structure of the existing one by iterating over its properties and copying them on the primitive level.
Like this:
Also we can use the method Object.assign for that.
The syntax is:
For instance, we can use it to merge several objects into one:
If the receiving object (
user ) already has the same named property, it will be overwritten:
We also can use
Object.assign to replace the loop for simple cloning:
It copies all properties of
user into the empty object and returns it. Actually, the same as the loop, but shorter.
Until now we assumed that all properties of
user are primitive. But properties can be references to other objects. What to do with them?
Like this: Pass generate no public key certificate.
Now it’s not enough to copy
clone.sizes = user.sizes , because the user.sizes is an object, it will be copied by reference. So clone and user will share the same sizes:
Like this:
To fix that, we should use the cloning loop that examines each value of
user[key] and, if it’s an object, then replicate its structure as well. That is called a “deep cloning”.
There’s a standard algorithm for deep cloning that handles the case above and more complex cases, called the Structured cloning algorithm. In order not to reinvent the wheel, we can use a working implementation of it from the JavaScript library lodash, the method is called _.cloneDeep(obj).
Summary
Objects are associative arrays with several special features.
They store properties (key-value pairs), where:
To access a property, we can use:
Additional operators:
Objects are assigned and copied by reference. In other words, a variable stores not the “object value”, but a “reference” (address in memory) for the value. So copying such a variable or passing it as a function argument copies that reference, not the object. All operations via copied references (like adding/removing properties) are performed on the same single object.
To make a “real copy” (a clone) we can use
Object.assign or _.cloneDeep(obj).
What we’ve studied in this chapter is called a “plain object”, or just
Object .
There are many other kinds of objects in JavaScript:
Object Keys Js
Js Object Key Value
They have their special features that we’ll study later. Sometimes people say something like “Array type” or “Date type”, but formally they are not types of their own, but belong to a single “object” data type. And they extend it in various ways.
Js Get Object Keys
Objects in JavaScript are very powerful. Here we’ve just scratched the surface of a topic that is really huge. We’ll be closely working with objects and learning more about them in further parts of the tutorial.
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