Table of Contents #
- Add a property to an Object in TypeScript
- Dynamically add Properties to an Object in TypeScript
- Set an Object's property name from a Variable in TypeScript
- Constructing the object's property name from multiple variables
- Using Object.assign() in TypeScript
Add a property to an Object in TypeScript #
To add a property to an object in TypeScript:
- Mark the property on the interface or type as optional.
- Use the interface to type the object.
- Add the property to the object.
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interface Person { name: string; age?: number; // 👈️ mark as optional so you can add it later}const obj: Person = { name: 'Bobby Hadz',};obj.age = 30;
We set the age property on the Person interface tooptional.
The age property can either be undefined or a number. This approach is used when you know the name of the property and the type of its value ahead of time as it keeps things type safe.
Now you can initialize the object without the property and set it later on.
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interface Person { name: string; age?: number; // 👈️ mark as optional}const obj: Person = { name: 'Bobby Hadz',};obj.age = 30;// ⛔ Error: Type 'string' is not assignable to type 'number'.obj.age = 'hello';
If you try to assign a non-numeric value to the age property, you'd get anerror.
Add any property to an object in TypeScript #
You can use theRecordutility type to add any property of any type to an object.
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const obj: Record<string, any> = {};// 👇️ can now add any property to the objectobj.name = 'Bobby Hadz';obj.age = 30;// 👇️ { name: 'Bobby Hadz', age: 30 }console.log(obj);
The Record utility type allows us to enforce the type of an object's values inTypeScript, e.g. type Animal = Record<string, string>.
The Record utility type constructs an object type, whose keys and values are of the specified type.
We used a type of any for the values and a type of string for the keys inthe object.
This is very broad and allows us to add any property of any type to theobject.
It's best to be as specific as possible when typing an object in TypeScript. We should try to leverage the language as much as possible.
Add any property but type key-value pairs you know in advance #
You can specify the properties and the types in an object that you know aboutand use the Record utility type to allow the user to add other properties.
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interface Animal extends Record<string, any> { name: string; age: number;}const obj: Animal = { name: 'Alfred', age: 3 };// 👇️ Can now add any property, but name and age are typedobj.type = 'Dog';
The interface we created requires the name and age properties but alsoextends a type that allows any string properties with values of any type.
This is better than just using the Record utility type with string keys andany values because we get type safety when it comes to the properties weexplicitly specified.
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interface Animal extends Record<string, any> { name: string; age: number;}const obj: Animal = { name: 'Alfred', age: 3 };// ⛔️ Type 'number' is not assignable to type 'string'.obj.name = 5;
Setting a property to the incorrect type causes an error because we've alreadydeclared the name property to be of type string in the Animal interface.
Dynamically add Properties to an Object in TypeScript #
Use an index signature to dynamically add properties to an object.
Index signatures are used when we don't know all of the names of a type'sproperties and the type of their values ahead of time.
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interface Person { [key: string]: any;}const obj: Person = {};obj.name = 'Bobby Hadz';obj.age = 30;
The {[key: string]: any} syntax is anindex signaturein TypeScript and is used when we don't know all the names of a type'sproperties and the shape of the values ahead of time.
The index signature in the examples means that when the object is indexed with a string, it will return a value of any type.
You might also see the index signature {[key: string]: string} in examples. Itrepresents a key-value structure that when indexed with a string returns avalue of type string.
Explicitly typing specific properties #
If the value of the string keys were set to any in the index signature, youcould add properties to the object of any type, since anything is more specificthan any.
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interface Person { [key: string]: any; age: number; name: string; country?: string;}const obj: Person = { name: 'Bobby Hadz', age: 30,};obj.country = 'Chile';obj.language = 'TypeScript';
This is a good way to narrow down the type of some of the properties that youknow ahead of time.
For example, if I try to set the age property to a string, the type checkerwould throw an error because it expects a number.
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interface Person { [key: string]: any; age: number; name: string; country?: string;}const obj: Person = { name: 'Bobby Hadz', age: 30,};// ⛔️ Error: Type 'string' is not assignable to type 'number'.obj.age = '100';
We've set the age property to have a type of number, so the type checkerhelps us spot an error in our application.
Using a union to dynamically add properties to an object #
You can also use a union type to dynamically add properties to an object.
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interface Person { // 👇️ key value [key: string]: string | number;}const obj: Person = { name: 'Bobby Hadz',};obj.age = 30;obj.country = 'Chile';
The index signature in the example means that when the object is indexed with astring, it will return a value of typestring or number.
The keys in the object can either have a type of string or number.
Both types are allowed and you can use the union | syntax to include as manytypes as necessary.
If we try to set a property with a different value on the object, we'd get anerror.
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interface Person { // 👇️ key value [key: string]: string | number;}const obj: Person = { name: 'Bobby Hadz',};// ⛔️ Type 'boolean' is not assignable to type 'string | number'.ts(2322)obj.isProgrammer = true;
Dynamically adding properties to an object with the Record type #
The Record utility type constructs an object type whose keys and values are ofa specified type.
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const obj: Record<string, any> = { name: 'Bobby Hadz',};obj.age = 30;obj.country = 'Chile';
We typed the object above to have keys of type string and values of typeany.
The first type we passed to the generic is the type for the Keys, and the second is the type for the values.
If you know the type of some of the values ahead of time, specify them in aninterface for better type safety.
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interface EmployeeData extends Record<string, any> { role: string; salary: number; color?: string;}const employees: Record<string, EmployeeData> = { tom: { role: 'accountant', salary: 10, color: 'blue' }, alice: { role: 'manager', salary: 15 },};// ⛔️ Type 'number' is not assignable to type 'string'.ts(2322)employees.tom.role = 100;// ✅ still worksemployees.tom.hello = 'world';
The interface EmployeeData extends from the Record constructed type withstring keys and any type values.
The interface sets a specific type for the 3 properties that we know about in advance.
If we try to set the role, salary or color properties to an incompatibletype, we'd get an error.
Set an Object's property name from a Variable in TypeScript #
Use computed property names to set an object's property name from a variable inTypeScript.
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type Person = { name: string; country: string;};const myVar = 'name';const obj: Person = { [myVar]: 'Bobby Hadz', country: 'Chile',};// 👇️ { name: 'Bobby Hadz', country: 'Chile' }console.log(obj);
We set an object's property name from a variable using computed properties.
All we had to do is wrap the variable in square brackets [] for it to get evaluated before being assigned as a property on the object.
Here is an example that uses a function.
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type Person = { name: string; country: string;};const myVar = 'name';function getObj(): Person { return { [myVar]: 'Bobby Hadz', country: 'Chile', };}const obj = getObj();// 👇️ {name: 'Tom', country: 'Chile'}console.log(obj);
Constructing the object's property name from multiple variables #
The expression between the square brackets gets evaluated, so you couldconstruct the object's property name by using multiple variables orconcatenating strings.
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const myVar = 'na';const myVar2 = 'me';// 👇️ const obj: { [x: string]: string; country: string;}const obj = { [myVar + myVar2]: 'Bobby Hadz', country: 'Chile',};// 👇 {name: 'Bobby Hadz', country: 'Chile'}console.log(obj);
You can also use a template literal.
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const myVar = 'na';const myVar2 = 'me';// 👇️ const obj: { [x: string]: string; country: string;}const obj = { [`${myVar}${myVar2}`]: 'Bobby Hadz', country: 'Chile',};// 👇 {name: 'Bobby Hadz', country: 'Chile'}console.log(obj);
But notice that TypeScript was not able to type the property in the object asname and instead typed it as a more generic index signature of [x: string](any string property).
If you are sure about the value the expression will evaluate to, use atype assertion.
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const myVar = 'na';const myVar2 = 'me';// 👇️ const obj: const obj: { name: string; country: string;}const obj = { [(myVar + myVar2) as 'name']: 'Bobby Hadz', country: 'Chile',};// 👇️ {name: 'Bobby Hadz', country: 'Chile'}console.log(obj);
Now the object is typed correctly and we can still use the dynamic nature of thecomputed property names feature.
Using Object.assign() in TypeScript #
To use the Object.assign() method in TypeScript, pass a target object as thefirst parameter to the method and one or more source objects.
The method will copy the properties from the source objects to the targetobject.
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const obj1 = { name: 'Bobby Hadz' };const obj2 = { country: 'Chile' };// 👇️ const result: { name: string; } & { country: string; }const result = Object.assign({}, obj1, obj2);// 👇️ { name: 'Bobby Hadz', country: 'Chile' }console.log(result);
We used theObject.assignmethod to merge two source objects into a target object.
The first parameter the method takes is a target object to which the properties of the source objects are applied.
The next parameters are source objects - objects containing the properties youwant to apply to the target.
Caveats around using an existing object as the target #
In the example, we passed an empty object as the target because you shouldgenerally avoid mutating objects as it introduces confusion.
For example, we could've passed obj1 as the target and obj2 as the source.
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const obj1 = { name: 'Bobby Hadz' };const obj2 = { country: 'Chile' };// 👇️ const result: { name: string; } & { country: string; }const result = Object.assign(obj1, obj2);// 👇️ {name: 'Bobby Hadz', country: 'Chile'}console.log(result);// 👇️ {name: 'Bobby Hadz', country: 'Chile'}console.log(obj1);// ⛔️ Error: Property 'country' does not// exist on type '{ name: string; }'.ts(2339)console.log(obj1.country);
Note that the target object is changed in place.
This is especially problematic when using TypeScript because obj1 was changedin place but its type is still {name: string}, even though the object containsa country property as well.
This is why you will often see an empty object passed as the first parameter to the Object.assign method.
The return type of Object.assign in TypeScript #
TypeScript uses anintersection typeto type the return value of the Object.assign() method.
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const obj1 = { name: 'Bobby Hadz' };const obj2 = { country: 'Chile' };// 👇️ const result: { name: string; } & { country: string; }const result = Object.assign({}, obj1, obj2);// 👇️ { name: 'Bobby Hadz', country: 'Chile' }console.log(result);
In other words, the return value has a type that has all of the members of allof the objects you passed to the Object.assign() method.
The latter object wins #
When you use the Object.assign() method with objects that have the sameproperties, the properties are overwritten by objects that come later in theparameter list.
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const obj1 = { name: 'Bobby Hadz' };const obj2 = { name: 'James' };// 👇️ const result: { name: string; } & { name: string; }const result = Object.assign({}, obj1, obj2);console.log(result); // 👉️ {name: 'James'}console.log(result.name); // 👉️ "James"
Both of the objects in the example have a name property, so the object that ispassed later in the parameters order wins.
An alternative to using Object.assign #
You should also consider using thespread syntax (...)as a replacement for Object.assign().
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const obj1 = { name: 'Bobby Hadz' };const obj2 = { country: 'Chile' };// 👇️ const result: {country: string;name: string;}const result = { ...obj1, ...obj2 };// 👇️ { name: 'Bobby Hadz', country: 'Chile' }console.log(result);
The spread syntax (...) unpacks the properties of the objects into a new object.
It is much easier for TypeScript to type the new object correctly than it is when you use the Object.assign() method.
This is generally a better approach because you can't shoot yourself in the footby forgetting to provide an empty object as the first parameter of theObject.assign() method.
You can unpack as many objects as necessary into a new object and if two objectshave the same property, the object that comes later wins.
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const obj1 = { name: 'Bobby Hadz' };const obj2 = { name: 'James' };// 👇️ const result: {name: string;}const result = { ...obj1, ...obj2 };console.log(result); // 👉️ {name: 'James'}
Both of the objects have a name property, so the latter object wins.
FAQs
How do I add a property to an object in TypeScript? ›
- Solution 1: Explicitly type the object at declaration time.
- Solution 2: Use an object index signature.
- Solution 3: Use the Record utility type.
- Solution 4: Use the Map data type.
- Solution 5: Consider an optional object property.
Some of the ways to add a property to an object JavaScript include: using dot notation, using bracket [ ] notation, using the defineProperty() method, using the spread operator, and using the Object. assign() method.
How do you write a property in TypeScript? ›First, we have to set a property into the object in a typescript for appending a property that is optional on the interface and is allocated to the object with the help of the '? ' keyword; after that, we can append the property at a time with no type error.
How to get value of object property in TypeScript? ›You can easily get an object's value by a key in Typescript using bracket notation, i.e., obj['key'] , obj[myVar] , etc. If the key exists, you will get the corresponding value back.
How do you set an object property with variables? ›- Example. const obj = {a: 'foo'} const prop = 'bar' // Set the property bar using the variable name prop obj[prop] = 'baz' console.log(obj);
- Output. This will give the output − { a: 'foo', bar: 'baz' } ...
- Example. ...
- Output.
Using the assign () method
In JavaScript, assign() is an in-built method. Using the assign() method, we can assign or add a new value to an existing object or we can create the new object without changing the existing object values.
- Create Objects in Different Ways.
- Clone a JS Object.
- Merge Property of Two Objects.
- Loop Through an Object.
- An attention-grabbing headline.
- A concise opening statement.
- A cleverly crafted narrative that describes the home's best features.
- A list of any special promotions.
- An enticing call to action.
Right-click and select Add New Properties File. A new properties file will be added to your project. The new file will be selected and highlighted in the list.
How do I set properties file? ›- Instantiate the Properties class.
- Populate the created Properties object using the put() method.
- Instantiate the FileOutputStream class by passing the path to store the file, as a parameter.
How do you extract property from an object? ›
Extracting a property
const { identifier } = expression; Where identifier is the name of the property to access and expression should evaluate to an object. After the destructuring, the variable identifier contains the property value. Try the demo.
To select objects from a collection, use the First, Last, Unique, Skip, and Index parameters. To select object properties, use the Property parameter. When you select properties, Select-Object returns new objects that have only the specified properties.
How do I add a key value to an existing object in TypeScript? ›Answer: Use Dot Notation or Square Bracket
You can simply use the dot notation ( . ) to add a key/value pair or a property to a JavaScript object.
Create Object using Object Literal Syntax
Define an object in the { } brackets with key:value pairs separated by a comma. The key would be the name of the property and the value will be a literal value or a function. Syntax: var <object-name> = { key1: value1, key2: value2,...};
A new deed must be written if an owner wants to include a co-owner in his ownership of the property. To be valid under the Transfer of Property Act, this new deed must also be recorded at the sub registrar's office.
How to get all properties of an object JavaScript? ›getOwnPropertyNames() The Object. getOwnPropertyNames() static method returns an array of all properties (including non-enumerable properties except for those which use Symbol) found directly in a given object.
What are the properties in an object? ›What is a property? A property is an attribute of an object or an aspect of its behavior. For example, properties of a document include its name, its content, and its save status, and whether change tracking is turned on. To change the characteristics of an object, you change the values of its properties.
How would you set or change the properties of a drawing object? ›- Open a drawing.
- Double-click the object that you want to modify. For example, double-click a part or a bolt, or a reinforcing bar.
- Change the desired properties.
- Give a new name for the properties file and save the file. ...
- If you want to apply the change in the object, click Modify.
...
Algorithm
- Step 1 − Define the key/s used in creating the object.
- Step 2 − Create an object and use the keys above defined.
- Step 3 − Apply JSON. stringify() on the object created above to display the object.
JavaScript methods are actions that can be performed on objects. A JavaScript method is a property containing a function definition. Methods are functions stored as object properties.
How do you create and modify an object in JavaScript? ›
- By object literal.
- By creating instance of Object directly (using new keyword)
- By using an object constructor (using new keyword)
We can also add a property to an object in an array of objects using the map method in JavaScript. The map() method creates a new array by calling a function on each element in the given array.
How to add key-value pair in JavaScript object dynamically? ›For adding an object key dynamically, use the bracket notation. First, store the key in a variable and then specify the variable name in bracket notation to set a key with value as a key-value pair in an object. This blog post defines the dynamic object key in JavaScript.
How to add dynamic key in object using JavaScript? ›If you're able to use ES6 JavaScript features, you can use Computed Property Names to handle this very easily: var key = 'DYNAMIC_KEY', obj = { [key]: 'ES6! ' }; console. log(obj); // > { 'DYNAMIC_KEY': 'ES6!
How do I modify an object key? ›To rename an object key in JavaScript, assign the value of the old key property to a new property with the new key, then delete the old key property. In this example, we create the new property by direct assignment to the object indexed with the new key. We delete the old key property with the delete operator.
How do I add a element to an array object in TypeScript? ›- export class Pixel { constructor(x: number, y: number) {} }
- this. pixels. push(new Pixel(x, y));
- var p = {x:x, y:y}; this. pixels. push(p);
- Use Object. keys to get all keys of the object.
- Apply any logic, to decide which values should be updated.
- Update the value of each using a loop like forEach or for .
- let div = document.createElement('div');
- let text = node.textContent;
- element.innerHTML = 'new content';
- parentNode.appendChild(childNode);
- parentNode.insertBefore(newNode, existingNode);
- parentNode.replaceChild(newChild, oldChild);
- let childNode = parentNode.removeChild(childNode);
Property is any item that a person or a business has legal title over. Property can be tangible items, such as houses, cars, or appliances, or it can refer to intangible items that carry the promise of future worth, such as stock and bond certificates.
What are the types of property? ›- Movable Property. Movable property can be moved from one place to another without causing any damage. ...
- Immovable Property. Immovable property is one that cannot be moved from one place to another place. ...
- Tangible Property. ...
- Intangible Property. ...
- Public Property. ...
- Private Property. ...
- Personal Property. ...
- Real Property.
What is property template? ›
A property template has no function or meaning in the object store until it is assigned to a class as a custom property. During property template creation, you assign values to metadata properties such as data type and cardinality (determines whether a property holds a single value or multiple values).
What does ${ mean in a properties file? ›The properties files are processed in the order in which they appear on the command line. Each properties file can refer to properties that have already been defined by a previously processed properties file, using ${varname} syntax.
How does property file work? ›PropertyFile is an online communications tool that connects you with your customers, allowing you to provide a better level of customer service, making your agency stand out from the crowd and allowing you to win more instructions.
What is property file? ›What is a Plot File? It is a real estate terminology, which serves as a documented promise of a piece of land have a certain area size in the planned or existing housing society or a scheme to a particular property investor.
Where are file properties? ›To view information about a file or folder, right-click it and select Properties. You can also select the file and press Alt + Enter . The file properties window shows you information like the type of file, the size of the file, and when you last modified it.
What is property in JavaScript? ›A JavaScript property is a member of an object that associates a key with a value. A JavaScript object is a data structure that stores a collection of properties. A property consists of the following parts: A name (also called a key), which is either a string or a symbol. A value, which can be any JavaScript value.
How do I find file properties? ›You can also Alt-click on a file or folder to access its properties. The General tab of the Properties dialog box will provide you with information such as the full path to the file or folder, its size, what application is configured to open it, and the date it was created, last modified, and accessed.
How to get specific values from object in JavaScript? ›To get only specific values in an array of objects in JavaScript, use the concept of filter().
How to copy some properties of object in JavaScript? ›Using Object.
assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object.
- Object.keys(obj) – returns all the keys of object as array.
- Object.values(obj) – returns all the values of the object as array.
- Object.entries(obj) – returns an array of [key, value]
How do you find out if a property is in an object? ›
We can check if a property exists in the object by checking if property !== undefined . In this example, it would return true because the name property does exist in the developer object.
How many types of object properties are there? ›There are two types of object properties: The data property and the accessor property.
Which method is used to assign the value to any property in an object? ›Object.assign() The Object.assign() static method copies all enumerable own properties from one or more source objects to a target object. It returns the modified target object.
How do you update values in object TypeScript? ›To update all values in an object, the easiest way is to: Use Object. keys to get all keys of the object. Apply any logic, to decide which values should be updated. Update the value of each using a loop-like forEach or for.
How to add property and value to object in JavaScript? ›In order to add a new property to an object, you would assign a new value to a property with the assignment operator ( = ). For example, we can add a numerical data type to the gimli object as the new age property. Both the dot and bracket notation can be used to add a new object property.
How do you add values in TypeScript? ›TypeScript - Array push()
push() method appends the given element(s) in the last of the array and returns the length of the new array.
- Example: Attach property to object. ...
- Example: prototype. ...
- Example: prototype. ...
- Example: Object's prototype. ...
- Example: Changing Prototype. ...
- function Student() { this.name = 'John'; this.
- Dot property accessor: object.property.
- Square brackets property accessor: object['property']
- Object destructuring: const { property } = object.
- Create Objects in Different Ways.
- Clone a JS Object.
- Merge Property of Two Objects.
- Loop Through an Object.
Using Property Accessors
Property accessors use dot notation or bracket notation to give access to an object's properties, which can also be called keys. You can use them to add a key-value pair to an object. It's the most straightforward way of adding a key-value pair to an object.
How do you add a key and a value to an object? ›
The most common and straightforward way to add a key-value pair to an object is to use the dot notation. You have probably already used this in the past, and it's sufficient in most situations you will encounter.
How can we add the custom property method to all instances of an object? ›Using the prototype object to add custom methods to objects
The prototype object can also help you quickly add a custom method to an object that is reflected on all instances of it. To do so, simply create the object method as usual, but when attaching it to the object (you guessed it), use "prototype" beforehand.
Prototype is a creational design pattern that allows cloning objects, even complex ones, without coupling to their specific classes. All prototype classes should have a common interface that makes it possible to copy objects even if their concrete classes are unknown.
What does .prototype do in JavaScript? ›Whenever we create a JavaScript function, JavaScript adds a prototype property to that function. A prototype is an object, where it can add new variables and methods to the existing object. i.e., Prototype is a base class for all the objects, and it helps us to achieve the inheritance.
How to add elements in object in JavaScript? ›Using square [] bracket: In JavaScript, we can use [] brackets to add elements to an object. This is another way to add an element to the JavaScript object.
How can I add a specific item from an array? ›If you need to add an element to the beginning of your array, use unshift() . If you want to add an element to a particular location of your array, use splice() . And finally, when you want to maintain your original array, you can use the concat() method.
How do you add an object value to an array? ›The push() function is a built-in array method of JavaScript. It is used to add the objects or elements in the array. This method adds the elements at the end of the array. "Note that any number of elements can be added using the push() method in an array."
How do you define an object property? ›Object properties are defined as a simple association between name and value. All properties have a name and value is one of the attributes linked with the property, which defines the access granted to the property. Properties refer to the collection of values which are associated with the JavaScript object.
What is a property on an object? ›A property is an attribute of an object or an aspect of its behavior. For example, properties of a document include its name, its content, and its save status, and whether change tracking is turned on. To change the characteristics of an object, you change the values of its properties.