What is the syntax for declaring a function type with an optional parameter in Typescript?

I have a function with optional parameters that I am passing down to another component.

execute = (option: string = 'default'): void => {
  // ...
}

In the receiving component, there is a property called executeFunction where I intend to assign this function. What should be the correct type for this property?

My Attempt

If I define the property as follows, I am unable to provide any arguments when calling it.

executeFunction: () => void;

If I define the property like this instead, I cannot invoke it without providing a parameter value.

executeFunction: (param: string) => void;

When defining the field like this, I compromise on type safety.

executeFunction: any;

Answer №1

Defining this can be done in the following way:

updateFunc: (flag?: boolean) => void;

This will result in the default value being assigned to the flag parameter when the function referenced as update is called.

Answer №2

You have the option to utilize the question mark.

function hello(name?: string): void {
    if (name) {
        console.log(`Greetings ${name}!`);
    } else {
        console.log(`I am unaware of your identity :(`);
    }
}

hello('J. Smith'); // Greetings J. Smith!
hello(); // I am unaware of your identity :(

If you require a default value, you can set it using assignment.

function hello(name: string = 'John Doe'): void {
    if (name) {
        console.log(`Greetings ${name}!`);
    } else {
        console.log(`I am unaware of your identity :(`);
    }
}

hello('J. Smith'); // Greetings M. Azyoksul!
hello(); // Greetings John Doe!

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