Js Dynamically Generate Object Key

Nov 19, 2019  To create a dynamic property on the object obj we can do: obj'propertyname' = 'somevalue'; what this does is, it creates a new property on the object obj which can be accessed as console.log(obj.propertyname); This will output the value somevalue on the console. Defining a dynamic property using Object.defineProperty.

1. Js Object Add Key
2. Js Dynamically Generate Object Key Value
3. Javascript Dynamic Object Key
4. Js Dynamically Generate Object Key Value
5. Js Object Key Value
6. Javascript Add Key To Object

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.

Object.keys, values, entries

For plain objects, the following methods are available:

• Object.keys(obj) – returns an array of keys.
• Object.values(obj) – returns an array of values.
• Object.entries(obj) – returns an array of [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:

Object.keys/values/entries ignore symbolic properties

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.

Transforming objects

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:

1. Use Object.entries(obj) to get an array of key/value pairs from obj.
2. Use array methods on that array, e.g. map.
3. Use 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.

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:

1. The first property has the name 'name' and the value 'John'.
2. The second one has the name '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.

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.

Js Object Add Key

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.

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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:

Js Dynamically Generate Object Key Value

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:

1. Modify the object’s behavior to treat __proto__ as a regular property. We’ll learn how to do it in the chapter Prototype methods, objects without __proto__.
2. Using Map data structure which supports arbitrary keys. We’ll learn it in the chapter Map and Set.

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:

Miracle box v2.5.4.0 key generator download 2017. 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:

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.

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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:

• USA (1) goes first
• then Switzerland (41) and so on.

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.

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:

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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 constcan 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:

• Arguments dest, and src1, ., srcN (can be as many as needed) are objects.
• It copies the properties of all 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:

Javascript Dynamic Object Key

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:

Js Dynamically Generate Object Key Value

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”.

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Summary

Objects are associative arrays with several special features.

They store properties (key-value pairs), where:

• Property keys must be strings or symbols (usually strings).
• Values can be of any type.

Js Object Key Value

To access a property, we can use:

• The dot notation: obj.property.
• Square brackets notation obj['property']. Square brackets allow to take the key from a variable, like obj[varWithKey].

Additional operators:

• To delete a property: delete obj.prop.
• To check if a property with the given key exists: 'key' in obj.
• To iterate over an object: 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).

Javascript Add Key To Object

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.
• …And so on.

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.