TypeScript Optional Parameters

Summary: in this tutorial, you will learn how to use the TypeScript optional parameters for functions.

In JavaScript, you can call a function without passing any arguments even though the function specifies parameters. Therefore, JavaScript supports the optional parameters by default.

In TypeScript, the compiler checks every function call and issues an error in the following cases:

• The number of arguments is different from the number of parameters specified in the function.
• Or the types of arguments are not compatible with the types of function parameters.

Because the compiler thoroughly checks the passing arguments, you need to annotate optional parameters to instruct the compiler not to issue an error when you omit the arguments.

To make a function parameter optional, you use the ? after the parameter name. For example:

function multiply(a: number, b: number, c?: number): number {

    if (typeof c !== 'undefined') {
        return a * b * c;
    }
    return a * b;
}Code language: JavaScript (javascript)

How it works:

• First, use the ? after the c parameter.
• Second, check if the argument is passed to the function by using the expression typeof c !== 'undefined'.

Note that if you use the expression if(c) to check if an argument is not initialized, you would find that the empty string or zero would be treated as undefined.

The optional parameters must appear after the required parameters in the parameter list.

For example, if you make the b parameter optional, and c parameter required the TypeScript compiler will issue an error:

function multiply(a: number, b?: number, c: number): number {

    if (typeof c !== 'undefined') {
        return a * b * c;
    }
    return a * b;
}Code language: JavaScript (javascript)

Error:

error TS1016: A required parameter cannot follow an optional parameter.

Summary

• Use the parameter?: type syntax to make a parameter optional.
• Use the expression typeof(parameter) !== 'undefined' to check if the parameter has been initialized.