Square brackets: jsObj'key' + i = 'example' + 1; In JavaScript, all arrays are objects, but not all objects are arrays. The primary difference (and one that's pretty hard to mimic with straight JavaScript and plain objects) is that array instances maintain the length property so that it reflects one plus the numeric value of the property whose name is numeric and whose value, when converted. Syntax 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. May 03, 2016 jQuery UI Widgets › Forums › Grid › Need to create grid columns dynamically from JSON Tagged: columns, grid, jqxgrid, json This topic contains 6 replies, has 4 voices, and was last updated by SysProfile 3 years, 11 months ago. 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.
Let’s step away from the individual data structures and talk about the iterations over them.
In the previous chapter we saw methods map.keys()
, map.values()
, map.entries()
.
These methods are generic, there is a common agreement to use them for data structures. If we ever create a data structure of our own, we should implement them too.
They are supported for:
Map
Set
Array
Plain objects also support similar methods, but the syntax is a bit different.
For plain objects, the following methods are available:
[key, value]
pairs.Please note the distinctions (compared to map for example):
Map | Object | |
---|---|---|
Call syntax | map.keys() | Object.keys(obj) , but not obj.keys() |
Returns | iterable | “real” Array |
The first difference is that we have to call Object.keys(obj)
, and not obj.keys()
.
Why so? The main reason is flexibility. Remember, objects are a base of all complex structures in JavaScript. So we may have an object of our own like data
that implements its own data.values()
method. And we still can call Object.values(data)
on it.
The second difference is that Object.*
methods return “real” array objects, not just an iterable. That’s mainly for historical reasons.
For instance:
Object.keys(user) = ['name', 'age']
Object.values(user) = ['John', 30]
Object.entries(user) = [ ['name','John'], ['age',30] ]
Here’s an example of using Object.values
to loop over property values:
Just like a for.in
loop, these methods ignore properties that use Symbol(..)
as keys.
Usually that’s convenient. But if we want symbolic keys too, then there’s a separate method Object.getOwnPropertySymbols that returns an array of only symbolic keys. Also, there exist a method Reflect.ownKeys(obj) that returns all keys.
Objects lack many methods that exist for arrays, e.g. map
, filter
and others.
If we’d like to apply them, then we can use Object.entries
followed Object.fromEntries
:
Object.entries(obj)
to get an array of key/value pairs from obj
.map
.Object.fromEntries(array)
on the resulting array to turn it back into an object.For example, we have an object with prices, and would like to double them:
It may look difficult from the first sight, but becomes easy to understand after you use it once or twice. We can make powerful chains of transforms this way.
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.
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:
'name'
and the value 'John'
.'age'
and the value 30
.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.
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:
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}
.
Codeigniter 3 hmac generate key generator. 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.
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 (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.
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.
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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:
__proto__
as a regular property. We’ll learn how to do it in the chapter Prototype methods, objects without __proto__.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.
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
:
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.
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:
Now it works as intended.
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.
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.
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.
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:
dest
, and src1, .., srcN
(can be as many as needed) are objects.src1, .., srcN
into dest
. In other words, properties of all arguments starting from the 2nd are copied into the 1st. Then it returns dest
.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:
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).
Objects are associative arrays with several special features.
They store properties (key-value pairs), where:
To access a property, we can use:
obj.property
.obj['property']
. Square brackets allow to take the key from a variable, like obj[varWithKey]
.Additional operators:
delete obj.prop
.'key' in obj
.for (let key in obj)
loop.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:
Array
to store ordered data collections,Date
to store the information about the date and time,Error
to store the information about an error.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.
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.