By 06/07/2015 11 Comments

TypeScript Optional Parameter List

Optional Parameter List:

  • In TypeScript, the all parameters of a function are required by default.
  • In JavaScript if we do not supply the full parameters to the functions then the parameter value will be undefined and it returns text “undefined”.
  • Using TypeScript language if we do not supply all parameters to a function the compiler will generate a compile-time error.
  • TypeScript has a “?” question mark symbol which indicates the optional parameters in a function.
  • By using question mark “?” symbol TypeScript compiler does not generate the compile-time error.
  • Through question mark symbol “?” we can define unlimited number of optional parameters in function parameter-list.
  • Remember, the optional parameters must be declared as the last parameters in function parameter-list.


/* A string type anonymous function declaration which has 4 parameters. Notice that last two parameters (php and html) are marked as optional by “?” question mark symbol. */
var pLanguages = function (asp: string, js: string, php?: string, html?: string): string {
    return (asp + " , " + js + " , " + php + " , " + html);

/* Call and print the above anonymous function, 2 parameters are passed from it. Other 2 are optional parameters. */
document.write(pLanguages("", "JavaScript"));

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