optional parameter
Annotate optional parameter with ?, optional parameters have to be at the end of the list of parameters:
function log(message: string, userId?: string){}
default parameter
Default parameters don’t have to be at the end of the list of parameters. Typescript can infer the parameter type from its default value, it’s not necessary to add explicit type annotations to the default parameters:
function log(message: string, userId="Not signed in"){}