Understanding Default Parameters in TypeScript

When working on larger projects, we often come across scenarios where a function requires default values for some of its parameters. This is where default parameters come in. TypeScript allows us to set default parameters on functions so that when a parameter is not provided, it automatically falls back to the default value.

In this article, we will discuss how to use default parameters in TypeScript and answer some common questions related to them.

Defining Default Parameters

To define a default parameter in TypeScript, we simply assign a value to a function parameter. The following example demonstrates this:

“`
function greet(name: string = ‘world’) {
console.log(`Hello, ${name}!`);
}

greet(); // Outputs: Hello, world!
greet(‘Jane’); // Outputs: Hello, Jane!
“`

In the above example, we defined a function `greet()` that takes a `name` parameter with a default value of `’world’`. When we call the function without passing any value for `name`, it defaults to `’world’`.

Overriding Default Parameters

We can also override default parameters by passing a value explicitly. The following example demonstrates this:

“`
function greet(name: string = ‘world’) {
console.log(`Hello, ${name}!`);
}

greet(‘John’); // Outputs: Hello, John!
“`

In the above example, we called the `greet()` function with an explicit value for `name`, which overrides the default value of `’world’`.

Using Optional Parameters with Default Parameters

We can also use default parameters with optional parameters. The following example demonstrates this:

“`
function greet(name?: string, greeting: string = ‘Hello’) {
console.log(`${greeting}, ${name ? name : ‘world’}!`);
}

greet(); // Outputs: Hello, world!
greet(‘Jane’); // Outputs: Hello, Jane!
greet(‘John’, ‘Hi’); // Outputs: Hi, John!
“`

In the above example, we defined a function `greet()` that takes an optional `name` parameter and a default `greeting` parameter of `’Hello’`. If the `name` parameter is not provided, it defaults to `’world’`. If the `greeting` parameter is not provided, it defaults to `’Hello’`.

Common Questions

Can I set a default value for any parameter in a function?

No, default values can only be set for parameters that come after all the required parameters. For example, in the following function, `name` is a required parameter and `age` is a default parameter:

“`
function greet(name: string, age = 18) {
console.log(`Hello, ${name}! You are ${age} years old.`);
}
“`

Can I pass `undefined` to a function with a default parameter?

Yes, you can pass `undefined` to a function with a default parameter, but it will still use the default value. For example, in the following function, `name` has a default value of `’world’`:

“`
function greet(name = ‘world’) {
console.log(`Hello, ${name}!`);
}

greet(undefined); // Outputs: Hello, world!
“`

Can I use rest parameters with default parameters?

Yes, rest parameters can be used with default parameters. The default value will apply to each omitted parameter in the rest parameters. For example, in the following function, `names` is a rest parameter with a default value of `[‘world’]`:

“`
function greet(…names: string[] = [‘world’]) {
console.log(`Hello, ${names.join(‘, ‘)}!`);
}

greet(); // Outputs: Hello, world!
greet(‘Jane’, ‘John’); // Outputs: Hello, Jane, John!
“`

Can I use default parameters with arrow functions?

Yes, default parameters can be used with arrow functions. The following example demonstrates this:

“`
const greet = (name: string = ‘world’) => console.log(`Hello, ${name}!`);

greet(); // Outputs: Hello, world!
greet(‘Jane’); // Outputs: Hello, Jane!
“`

What happens if I pass `null` to a function with a default parameter?

If you pass `null` to a function with a default parameter, it will use `null` as the value. For example, in the following function, `name` has a default value of `’world’`, but if `null` is passed, it will use `null` as the value:

“`
function greet(name = ‘world’) {
console.log(`Hello, ${name}!`);
}

greet(null); // Outputs: Hello, null!
“`

Conclusion

Default parameters in TypeScript allow us to provide default values for function parameters. We can set default values, override them, use them with optional parameters, rest parameters, and arrow functions. Understanding default parameters can help us write more flexible and reusable code in our projects.

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