TypeScript Functions Optional and Default Parameters


Optional Parameters

In TypeScript, every parameter is assumed to be required by the function. You can add a ? at the end of a parameter name to set it as optional.

For example, the lastName parameter of this function is optional:

function buildName(firstName: string, lastName?: string) {
    // ...

Optional parameters must come after all non-optional parameters:

function buildName(firstName?: string, lastName: string) // Invalid

Default Parameters

If the user passes undefined or doesn't specify an argument, the default value will be assigned. These are called default-initialized parameters.

For example, "Smith" is the default value for the lastName parameter.

function buildName(firstName: string, lastName = "Smith") {
    // ...
buildName('foo', 'bar');      // firstName == 'foo', lastName == 'bar'
buildName('foo');             // firstName == 'foo', lastName == 'Smith'
buildName('foo', undefined);  // firstName == 'foo', lastName == 'Smith'