How To Use A TypeScript Interface?

An interface is a blueprint for an Object. It tells you which properties there are, which are required and optional and which type of data they hold.

TypeScript has become more popular than ever. For me it was not love at first sight, but now we are connected. I don't start a project without TypeScript! 😅 So if you experience the same feelings, your not alone 🤗

Most of the models in TypeScript are a combination of interfaces and classes. An interface is a blueprint of a class or object. In this IPizza interface, we define all the properties a pizza has. In each property, we define what kind of data type the information is.

Every property that is defined in an interface is required. If you want to make it optional, you must use the ? . For example, propertyName?: string if we define this property in an interface, it's optional. TypeScript won't give you an error if the property is missing in an Object. On the other hand, if a property is required, it will give an error if it is missing.

When a property is not defined in an interface you will get an error from the TypeScript compiler because the data is not according to the blueprint.

Example

We can all come up with properties for a pizza.

• Name
• Slices (the number of slices)
• Toppings
• Price
• Cheesecrust
• Vegan
• Vegetarian

Lets put them in the interface and decide what kind of data type they are.

interface IPizza {
    name: string;
    slices: number;
    toppings: string;
    price: number;
    cheescrust: boolean;
    vegan?: boolean;
    vegetarian?: boolean;
}

The example above shows an interface for our pizza. We gave all the properties a single data type. Now we can create our Pizza object and use the interface to ensure it has the correct properties.

const pizza: IPizza {
    name: 'Pizza BBQ',
    slices: 6,
    toppings: 'Tomato sauce, BBQ sauce',
    price: 15,
    cheescrust: true
}

Now the pizza is according to the interface. The interface is now a form of data validation. If we would add properties that are not in the interface or properties with wrong data types, the TypeScript will give errors.

const pizza: IPizza {
    name: 'Pizza BBQ',
    slices: 6,
    toppings: ['Tomatosauce', 'BBQ sauce'],
    price: 15,
    cheescrust: true,
    meat: true
}

With this object, you will get errors! 👇 (Check it onCodeSandbox for yourself)

Multiple values

But what if we want an array of strings or numbers to give our toppings or sizes? We can do that pretty quickly; write string[] or number[] in the interface.

interface IPizza {
    name: string;
    slices: number;
    toppings: string[];
    price: number;
    cheescrust: boolean;
    sizes: number[];
    vegan?: boolean;
    vegetarian?: boolean;
}

Now our pizza object is valid.

const pizza: IPizza {
    name: 'Pizza BBQ',
    slices: 6,
    toppings: ['Tomatosauce', 'BBQ sauce'],
    price: 15,
    cheescrust: true,
    vegan: false,
    vegetarian: false,
    sizes: [0, 1, 2, 3, 4]
}

If we want to type an Array with multiple pizza objects, we can do that the same way with IPizza[].

const pizzaArray: IPizza[] = []

Conditional values

Sometimes, you will say that a property can be a string or a null. In those cases, we use the pipe | to show that it is both.

// IPizza properties with an array of values.
interface IPizza {
    name: string;
    slices: number;
    toppings: string[];
    price: number;
    cheescrust: boolean | null;
    sizes: number[];
    vegan?: boolean;
    vegetarian?: boolean;
}

For example, with the cheescrust it is optional but can be a boolean or null.

const pizza: IPizza {
    name: 'Pizza Tuna',
    slices: 8,
    toppings: ['Tomatosauce'],
    price: 11.99,
    cheescrust: null,
    vegan: false,
    vegetarian: false,
    sizes: [0, 1, 2, 3, 4]
}

So with the Pizza Tuna, we say we dont want to offer cheese crust, so we give it a value of null.

Thanks!

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