Why are optional members utilized in a TypeScript interface?

Question

Why are optional members utilized in a TypeScript interface?

Currently, I am engaged in an online tutorial on typescript (although it is not in English, I will translate the example):

interface Message { 
    email: string; 
    receiver?: string; 
    subject?: string; 
    content: string; 
}

The concept of the ? in this context is to denote certain properties as optional. This raises a question that my tutorial does not explore - if interfaces in TypeScript are meant to serve as contracts ensuring specific properties are always present, what purpose do optional properties serve? After all, any additional or optional properties could simply be handled at the object or class definition level.

For instance, I can see the practicality of optional methods in Java interfaces due to their default method bodies that can be reused in implementing classes. However, the idea of optional properties in TypeScript seems somewhat perplexing upon initial examination.

typescript interface member

Answer 1

Answer №1

Optional members are properties that are not required to be present.

For instance, let's take a look at this function:

function send(msg: Message): void {
    // Check if the missing properties are present, and provide defaults as needed
    if(!msg.subject) msg.subject= 'no subject';
    if(!msg.receiver) msg.subject= 'no subject';

    console.log(`Subject: ${msg.subject}
    To: ${msg.receiver}
    From: ${msg.email}
    Content: ${msg.content}
    `)
}

The following calls are considered valid:

 // Required properties are provided, fulfilling the contract 
send({ email: '', content: ''})

// Required properties are provided, fulfilling the contract
send({ email: '', content: '', receiver:'' }) 

 //Invalid - missing required property
send({ email: '' }) 

// Invalid - excess properties included (only applies to object literals directly assigned to the parameter)
send({ email: '', content: '', Receiver: ''})

The function can access any field of the interface, but optional fields do not need to be specified during the call. When passing an object literal, the compiler ensures that only interface properties are used, with required properties included, and without mandating optional properties.

It is important to note that interfaces do not necessarily have to be implemented by classes; in the provided example, interfaces define the structure of objects. Whether dealing with classes or object literals, specifying only the necessary fields and adding optional fields as necessary proves to be quite beneficial.

Answer 2

Optional members are properties that are not required to be present.

For instance, let's take a look at this function:

function send(msg: Message): void {
    // Check if the missing properties are present, and provide defaults as needed
    if(!msg.subject) msg.subject= 'no subject';
    if(!msg.receiver) msg.subject= 'no subject';

    console.log(`Subject: ${msg.subject}
    To: ${msg.receiver}
    From: ${msg.email}
    Content: ${msg.content}
    `)
}

The following calls are considered valid:

 // Required properties are provided, fulfilling the contract 
send({ email: '', content: ''})

// Required properties are provided, fulfilling the contract
send({ email: '', content: '', receiver:'' }) 

 //Invalid - missing required property
send({ email: '' }) 

// Invalid - excess properties included (only applies to object literals directly assigned to the parameter)
send({ email: '', content: '', Receiver: ''})

The function can access any field of the interface, but optional fields do not need to be specified during the call. When passing an object literal, the compiler ensures that only interface properties are used, with required properties included, and without mandating optional properties.

It is important to note that interfaces do not necessarily have to be implemented by classes; in the provided example, interfaces define the structure of objects. Whether dealing with classes or object literals, specifying only the necessary fields and adding optional fields as necessary proves to be quite beneficial.

Why are optional members utilized in a TypeScript interface?

Answer №1

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