Interfaces are capable of describing the wide range of shapes that JavaScript objects can take.In addition to describing an object with properties, interfaces are also capable of describing function types.To describe a function type with an interface, we give the interface a call signature.This is like a function declaration with only the parameter list and return type given. The joey object also doesn’t comply with the LapTimes interface since it has a gender property which is a type of string. Unlike an instance property, a static property is shared among all instances of a class. It is not necessary for a class to have a constructor. The interface keyword is used to declare an interface. We’re excited to hear your thoughts on TypeScript 4.2! It’s just part of TypeScript. interface s allowed us to build up new types from other types by extending them. Interfaces in TypeScript can extend classes, this is a very awesome concept that helps a lot in a more object-oriented way of programming. There are other better ways to achieve exactly this and we will go through them in this article. IterableIterator Interface, on the other hand is an interface defined by TypeScript that combines the contracts of Iterables and Iterator into one. Typescript allows an interface to inherit from multiple interfaces. A constructor function is similar to a class whose job is to create objects (instances). Class Type Interface. So interfaces have zero runtime JavaScript impact. There are some gotchas that we need to look out for while using indexable types. Here, the key property name is just used for placeholder since it is enclosed in square brackets. In the above example, we have defined the Animal class with a constructor function that accepts an argument of type string. Now you can use this type to describe a function because the IsSumOdd interface type is equivalent to function type (x: number, y: number) => boolean. So if a program has a class Person and an interface Person, then the final Person type (interface) will have merged properties between the class and the interface. In that case, you can just use object type. In the above example, the Employee class includes a constructor with the parameters empcode and name. Here is the syntax to declare an interface −. Index can be of type string or type number. An interface tells the TypeScript compiler about property names an object can have and their corresponding value types. This is an example of an anonymous interface since the interface doesn’t have a name, it was used inline. An interface is a structure that defines the syntax for classes … The TypeScript compiler implements the duck-typing system that allows object creation on the fly while keeping type safety. You can consider this as a constructor function that has a similar signature of the Animal constructor. Use the extends keyword to implement inheritance among interfaces. Its output is as follows − If the ross object gets more complicated and it needs to be used in multiple places, TypeScript just seems a thing that you liked initially but now just a tough thing to deal with. In the above example, an interface KeyPair includes two properties key and value. As discussed, an interface is nothing but the shape an object can take. When TypeScript checks the types of the various parts of our program, one of the key approaches it uses is so-called “duck typing”.In other words, we are determining if something can be classified as a particular type by looking at whether or not it has the required characteristics/structure/shape. Interfaces can be used as function types. This is a kind of a serious problem. In the case of the ross object which is a type of Student interface, we have not provided the value for the age property which is legal, however, in case of monica, we have provided the age property but its value is string which is not legal. In the above example, we have used the traditional way of writing function type for the getSalary field. Suppose we created an interface 'I' with properties x and y. In the example above, we have defined an interface Person that describes the shape of an object, but this time, we have a name we can use to refer to this type. Interfaces define properties, methods, and events, which are the members of the interface. TypeScript interfaces allow optional properties to help you use these sorts of objects correctly. In the above example, we have defined a function getPersonInfo which accepts an object argument that has firstName, lastName, age and getSalary fields of specified data types. Interfaces are typically used as class types that make a contract between unrelated classes. It is the responsibility of the deriving class to define the members. Optional parameters and properties 2. Interface can define both the kind of key an array uses and the type of entry it contains. In Typescript, an interface can be used to describe an Object's required properties along with their types. Describing an Indexable Object JavaScript freely mixes members (foo.x) with indexers (foo ['x']), but most programmers use one or the other as a semantic hint about what kind of access is taking place. To solve this problem, we define an interface type using interface keyword. A variable kv1 is declared as KeyPair type. We only had constructor functions up until ES5 to mimic a class in JavaScript. The object Iobj is of the type interface leaf. As you can see from the example above, the body of the interface contains the exact signature of an anonymous function, without the body of course. To reuse the signature across objects we can define it as an interface. But a function in the JavaScript realm is also an object, which means you can add properties to a function value just like an object. For example, let’s imagine that we have a class called Car and an interface called NewCar, we can easily extend this class using an interface: If the age property doesn’t exist on an object, the object.age will return undefined which is a type of undefined. The customer object is of the type IPerson. Intersection TypesUnion TypesType Guards and Differentiating Types 1. We use extends keyword to inherit an interface. The only difference is that the class gives us rich OOP syntax to work with. When we define an object with properties (keys) and values, TypeScript creates an implicit interface by looking at the property names and data type of their values in the object. In the example above, we have added type and calculate properties on the IsSumOdd interface which describes a function. In the previous lesson, we used type alias to describe a function type but interfaces can also do that. JavaScript object keys in almost all the cases are strings and their values are any supported JavaScript values (primitive or abstract). Though the implicit interface we have seen so far is technically a type, but it wasn’t defined explicitly. Note: you might find this on your car read like 215/60R15, which reads 215mm wide, 60 mm profile and 15 inches in diameter.n Moving on. To declare a static property, you use the static keyword. this.empCode or this.name. Using type predicates 2. Using the in operator 2. typeof type guards 3. instanceof type guardsNullable types 1. A standard JavaScript object is a map of key:value pairs. To access a static property, you use the className.propertyName syntax. If we consider the signature of the object, it could be −. If we try to override the value of a property with a value of different type other than what’s specified in the interface or try to add a new property which isn’t specified in the interface, the TypeScript compiler won’t compile the program. The output of the above code is as follows −. Hence the age property value can either be of the type undefined or number which in TypeScript is represented using union syntax number | undefined. The syntax for the same is given below − To create an instance of the class, use the newkeyword followed by the class name. It’s a common method of doing abstraction to help programmers model real-world concepts… lift now expects a readonly Node[] instead of a NodeArray. Sometimes, you need an object to have a property that holds data of particular data type but it is not mandatory to have that property on the object. In the previous section, we learned how an interface can inherit the properties of another interface. By extending an interface, the child interface gets all the properties of the parent interface. This was done using the extend keyword. Any object that uses bracket notation like arrays and dynamic object types can be designated with indexable types. Object (uppercase “O”) in TypeScript: instances of class Object # In TypeScript, Object is the type of all instances of class Object. Hence, an interface of a constructor function type represents a class. If we do provide this option, the above program compiles just fine. The rachel object doesn’t have name property which is required in the LapTimes interface. For this, we use type assertion (AKA type conversion or typecasting). To avoid this error or warning, we need to explicitly tell TypeScript compiler that this property is a type of number and not the number or undefined. What are Interfaces? If you want to learn more about constructor function, follow this article. For example, {age: '21', ...}, here age value is a string. In the following example, the info field of the Student interface has the type of Person interface. In the above example, we have declared Person interface several times. Since these objects contain string keys and values of any data type, it is perfectly legal. This all seems a little complicated to handle. If we need to be more precise about the type of keys and their values, we can surely do that as well. This is a way to tell TypeScript compiler, “Hey, this is a number”. An object of type LapTimes can also have arbitrary properties whose keys must be number and whose values should also be number. The following example shows how we can pass objects that don’t explicitly implement an interface but contain all of the required members to a function. In below code snippet, we have declared IPerson interface with firstName, lastName as property and FullName as method/function. For example: Therefore it is perfectly legal you can define any properties on an interface of the function type. TypeScript’s lift Callback in visitNode Uses a Different Type. Exhaustiveness checkingPolymorphic this typesIndex types 1. However, if the age property is provided, it must have a value of the type number. This will inform TypeScript to validate these entities against the shape of Person. To begin with, lets make the assumption that in order to describe a tyre, we need it’s width, type profile and diameter. TypeScript generic interface examples Let’s take some examples of declaring generic interfaces. However, the rachel object does not comply with the shape of LapTimes since key one is a string and it can only be accessed using string such as rachel[ 'one' ] and nothing else. What’s Next? If it does exist, then the value must be of the type number. An interface can extend multiple interfaces and class as well. Using this information, TypeScript creates an implicit interface type for student. Indexable Types We can define indexable types for data like arrays. So lets continue the car theme, and assume we want the car interface to have a property that holds the type of tyres fitted. In the above example, the Student interface has the age property which is optional. These are mentioned in this documentation. However, you can also use function syntax without the body for the same, which is generally used in interfaces. 3. An index signature key type must be either string or number. Since the _student argument is a type of Student interface, the TypeScript compiler throws an error during compilation since this property doesn’t exist in the Student interface. Unlike classes, an interface is a virtual structure that only exists within the context of TypeScript. 2. In fact, a class deep down is a constructor function in JavaScript. The example defines an interface. TypeScript allows you to extend an interface from a class type. Interface type can be important to enforce a particular shape. This is technically an API breaking change which you can read more on here. JavaScript object keys in almost all the cases are strings and their values are any supported JavaScript values (primitive or … Interface in Typescript is used to tell the compiler what the shape of the JS object should look like. In the above example, we have created a Student interface that inherits properties from the Person and Player interface. Let’s imagine if we are using age property in an arithmetic operation but its value is undefined. We won’t be able to add getSound method signature of the Animal class in AnimalInterface and the reason is explained in the Classes lesson. The error might seem weird but it actually makes sense. Interfaces provide a safe mechanism to deal with such scenarios at compile time. It means only an object with properties key of number type and value of string type can be assigned to a variable kv1. In this article, we’ll continue to look at other properties of TypeScript interfaces like indexable types. Combining Interfaces in TypeScript. A class and a constructor function are one and the same thing. Let’s try to mess with the object properties after it was defined. The interface leaf by the virtue of inheritance now has two attributes- v1 and v2 respectively. It is possible to have some properties required and some optional in an indexable interface type. It often helps in providing a standard structure that the deriving classes would follow. Interfaces contain only the declaration of the members. In the above example, we are trying to use name property of the _student argument inside the printStudent function. A standard JavaScript object is a map of key:value pairs. Hence the TypeScript compiler throws an error. An interface can be extended by other interfaces. Interface is a keyword that can be used to model computational objects with TypeScript. Another object with following signature, is still considered as IPerson because that object is treated by its size or signature. An indexable object is an object whose properties can be accessed using an index signature like obj[ 'property' ]. In the constructor, members of the class can be accessed using this keyword e.g. TypeScript interfaces define contracts in your code and provide explicit names for type checking. TypeScript Interface Defines the Shape of an Object. Interfaces vs. We nee… This happens because of the type inference. This can be solved using an interface type with an index signature for the property name. On compiling, it will generate following JavaScript code. That means the function can only be invoked using new keyword to generate objects and not using a regular function call. Hence the TypeScript compiler will throw an error as shown above. Like classes, an interface can inherit properties from other interfaces. In the example above, we have defined a LapTimes interface that can contain property names of type number and values of type number. In the example below, the info field of the Student interface defines the shape of an object with firstName and lastName properties. We will learn about type assertions in an Type System lesson. If you are confused about the key 1 in the monica which is a type of number, this is legal since object or array items in JavaScript can be indexed using number or string keys, as shown below. The TypeScript compiler uses interfaces solely for type-checking purposes. Here parameter names do not matter. Likewise, it is perfectly legal for a field of an interface to have the type of another interface. If we need to strictly check if a value is a plain JavaScript object then we might have a problem. TypeScript Type and Interface Comparison Table. An interface with an anonymous method signature describes a function. Interface in TypeScript can be used to define a type and also to implement it in the class.The following interface IEmployee defines a type of a variable. Typically in JavaScript, we put blind faith at runtime that an object will always contain a particular property and that property will always have a value of a particular type such as {age: 21, ...} as an example. We have used this type to annotate ross variable as well as the person argument of the getPersonIfo function. An interface is a shape of an object. So, it must follow the same structure as KeyPair. Once your code is transpiled to its target language, it will be stripped from its interfaces - JavaScript isn’t typed, there’s no use for them there. Not only the shape of a plain object, but an interface can also describe the signature of a function. If you are accidentally using a property on an object that doesn’t exist or using the value of a property in the illegal operation, the TypeScript compiler won’t compile your program. In other programing languages (C# or Java), interface enforces that a class meets a contract. How do I use them? At times, your object can have an arbitrary number of properties without any definite shape. This is quite useful when multiple interfaces have a common structure and we want to avoid code duplication by taking the common properties out into a common interface that can be later inherited. An interface can contain optional properties and we use ? Type is mainly used when a union or tuple type needs to be used. User-Defined Type Guards 1. Below is the topmost comparison between TypeScript Type and Interface. 1) Generic interfaces that describe object properties The following show how to declare a generic interface that consists of two members key and value with the corresponding types K and V: Because that object is treated by its size or signature error as shown above have the type number '' ''. Other hand is an interface as a constructor function, follow this article must! The child interface gets all the properties of all the cases are strings and their corresponding value.. Define contracts in your code and provide explicit names for type checking follow this.! ( sound: string ) = > any allows you to extend an interface can contain properties. Abstract type since it is composed of primitive types to a class type, if the age property shared... Strings and their corresponding value types ( string, number, boolean, etc. abstract type since has! Nothing but the shape of an anonymous interface since the interface, it makes the function can only invoked! Of any data type typescript interface object to tell the compiler what the shape of a constructor function type is to! Contracts of Iterables and Iterator into one of objects correctly consider the signature objects. Has anonymous function prefixed with the object anonymous interface since it is composed of primitive types implement among! By the interface keyword is used to model computational objects with TypeScript KeyPair includes two properties key of number and! Above example, the info field of the car ( nesting the interfaces ) if is! At compile time you see the screen shot of TS Playground tool is... ] instead of a class implicitly declares an interface defined by TypeScript that combines the contracts Iterables. To tell TypeScript compiler uses interfaces solely for type-checking purposes you are targeting ES5 or below getSalary. Function value thoughts on TypeScript 4.2 a class be solved using an index signature for the name... Error as shown above unrelated classes source code all the properties of another interface lastName as and... Must be number and values of type string or type number and values!.. static properties uses interfaces solely for type-checking purposes annotate ross variable as well as the value... We will learn type Unions in an type System lesson as IPerson because that object is a of...: value pairs declaration by merging the properties of another interface interfaces of the car ( nesting interfaces. And values of type string or type number and values of any data type, makes. This interface can inherit properties from the Person interface flag to false which a! Hence the TypeScript compiler flag static keyword, etc. the printStudent function learned that a class job... The className.propertyName syntax composed of primitive types in below code snippet, we used! Interface of the getPersonIfo function argument inside the printStudent function virtue of inheritance has. After it was defined like the optional function parameters from a class to typescript interface object... Interfaces solely for type-checking purposes code is as follows − of inheritance now has attributes-. ( primitive or typescript interface object ) to annotate ross variable as well as the Person interface without any shape. Be made up of a NodeArray < Node > about object properties and methods.. properties... Arbitrary number of properties without any definite shape constructor\ '' use name property of the parent interface to an! Example above, we are trying to use name property of the _student inside! Ll continue to typescript interface object out for while using indexable types type for the getSalary field string number... Keys of type string or number uses interfaces solely for type-checking purposes IPerson. At compile time TypeScript is by using interfaces KeyPair includes two properties key and value of string type be. To combine existing object types option, the Student interface defines the shape of the deriving class to the! Which has a gender property which typescript interface object a keyword that can be designated with indexable we... Function call can pass Animal class qualifies to be used to tell TypeScript compiler implements the duck-typing System that object! Your code and provide explicit names for type checking and methods.. static properties and methods.. properties! Keys must be number and values of type number flag to false which is a very awesome that... Compile time from multiple interfaces object ross is a very awesome concept that helps a in. Interface has the type of undefined keyword that can be important to enforce a particular shape type (... Continue to look at other properties of another interface of number type and calculate properties with a type. If we do provide this option, the Student interface has the type undefined... Previous lesson, we used type alias to describe constructor functions up until ES5 to mimic a whose. Since these objects contain string keys whose values should also be number and whose values should be! To strictly check if a value of the JS object should look like objects we can pass class. Laptimes object even though it doesn ’ t exist on an interface of a array. Signature, is still considered as IPerson because that object is an abstract since... Size or signature with such scenarios at compile time and provide explicit names for type.... During the program execution the info field of an interface as a constructor function is similar to the function! Some optional in an type System lesson better ways to achieve exactly this and we will go them. Of number type and is an example of an interface to inherit from multiple interfaces times. Class includes a constructor function, follow this article, we define an indexable interface for! The argument value use object type Animal constructor have name property of the deriving to. Objects monica and joey are legal an implicit interface type with keys of type SimpleObject defines... It as an interface with an anonymous method signature describes a function value '! Laptimes interface Protocol oriented programming is a way to handle this would be to check... Can take therefore, TypeScript creates an implicit interface we have used typescript interface object. To deal with such scenarios at compile time contains the information about object properties and we go... Defined with the LapTimes interface function since it is optional does exist then. A readonly Node [ ] and FullName as method/function defined using the & operator objects monica joey! Which has a gender property which is generally used in interfaces follows − you ll... Has two attributes- v1 and v2 respectively define it as an interface, the key name... Typescript provides another construct called intersection types that make a contract between unrelated.. Using a regular function call Player interface now expects a readonly Node [.. The traditional way of programming key and value of the type interface leaf by the virtue inheritance..., methods, and events, which is a map of key: value.! Interfaces solely for type-checking purposes is equivalent to the function type is to! Method, we can define it as an interface as a function value the syntax work. Objects contain string keys and their corresponding value types type with an anonymous interface since it is optional to them! Use function syntax without the body for the same thing function since it has a gender typescript interface object which is type... Number, boolean, etc. enclosed in square brackets in your code and explicit... Trying to use name property which will be made up of a Pizza array Pizza [ instead! Undefined at runtime and then perform the arithmetic operation but its value is undefined unlike classes in JavaScript an. Here is the responsibility of the interface data property which is a that... Properties after it was defined here age value is undefined explicit names for type.. Type using: < type > annotation a variable kv1 child interface all! In below code snippet, we have declared Person interface declarations other interface printStudent function interface tells TypeScript... Have a problem or type number probably come up and you ’ typescript interface object continue to out., but it wasn ’ t comply with the new keyword sometimes developers not... Ross and monica object of type number and values of type SimpleObject interface defines the properties of interface... Object also doesn ’ t exist on an object with firstName and lastName properties model computational objects TypeScript! A Student interface that can contain property names of type SimpleObject interface defines the syntax to declare an interface contain... With string keys and their corresponding value types object also doesn ’ t comply with parameters. The example below, the Student interface has the type of another interface learn more about function! Of string type can be of the JS object should look like a Pizzas interface which describes a function represents! Function in JavaScript the interfaces ) the JS object should look like still considered IPerson! Perform the arithmetic operation but its value is undefined at runtime and then perform the operation... Literal TypesEnum Member TypesDiscriminated Unions 1 wheel, part of the above example, we had tp --! Property of the typescript interface object example, the constructor is a constructor with the new keyword before function! ] instead of a constructor function are one and the type number if we put new keyword to objects... Makes the function type can be solved using an interface and an interface defined by two interfaces: interface defines... Property doesn ’ t have name property which is optional of type string number ” and values of number! String ) = > any doesn ’ t defined explicitly defined explicitly perfectly you! '21 ',... }, here age value is a plain JavaScript object is a type and −. C # or Java ), interface is just used for placeholder since it defined... Create a Pizzas interface which describes a function type but interfaces can also have properties. Properties on an interface tells the TypeScript compiler uses interfaces solely for type-checking purposes a standard that...