How to define function type with optional parameter in Typescript?

Question

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

update = (flag: boolean = true): void => {
  // ...
}

In the component that I pass this down to, I have a field called updateFunc that I want to assign this function. What should be the type of this field?

What I Tried

If I define the field like the following, I cannot call it with a parameter.

updateFunc: () => void;

If I define the field like the following, I cannot call it without a parameter.

updateFunc: (flag: boolean) => void;

If I define the field like the following, I lose the type safety.

updateFunc: any;

@pzaenger This works in my testing. Should be an answer. Thank you. — M. Akar
– M. Akar, Commented May 2, 2022 at 23:47

Mich · Accepted Answer · 2022-05-02 23:56:48Z

4

You can define this:

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

then the default value will be assigned to the flag parameter when the referenced function (update) will be executed.

answered May 2, 2022 at 23:56

Mich

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Comments

normeno · Accepted Answer · 2022-05-02 23:55:51Z

1

You can use the question mark.

function hello(name?: string): void {
    if (name) {
        console.log(`Hi ${name}!`);
    } else {
        console.log(`I don't know your name :(`);
    }
}

hello('M. Azyoksul'); // Hi M. Azyoksul!
hello(); // I don't know your name :(

If you need a default value, you can use assignment

function hello(name: string = 'masked man'): void {
    if (name) {
        console.log(`Hi ${name}!`);
    } else {
        console.log(`I don't know your name :(`);
    }
}

hello('M. Azyoksul'); // Hi M. Azyoksul!
hello(); // Hi masked man!

answered May 2, 2022 at 23:55

normeno

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