What is the question mark for in a Typescript parameter name

Question

It is to mark the parameter as optional.

• TypeScript handbook https://www.typescriptlang.org/docs/handbook/2/functions.html#optional-parameters
• TypeScript Deep Dive https://basarat.gitbook.io/typescript/type-system/functions#optional-parameters

This is to make the variable of Optional type. Otherwise declared variables shows "undefined" if this variable is not used.

export interface ISearchResult {  
  title: string;  
  listTitle:string;
  entityName?: string,
  lookupName?:string,
  lookupId?:string  
}

parameter?: type is a shorthand for parameter: type | undefined

So what is the difference? Question mark means "optional".
More precisely parameter?: type is equal parameter: type | undefined = undefined

The ? in the parameters is to denote an optional parameter. The Typescript compiler does not require this parameter to be filled in. See the code example below for more details:

// baz: number | undefined means: the second argument baz can be a number or undefined

// = undefined, is default parameter syntax, 
// if the parameter is not filled in it will default to undefined

// Although default JS behaviour is to set every non filled in argument to undefined 
// we need this default argument so that the typescript compiler
// doesn't require the second argument to be filled in
function fn1 (bar: string, baz: number | undefined = undefined) {
    // do stuff
}

// All the above code can be simplified using the ? operator after the parameter
// In other words fn1 and fn2 are equivalent in behaviour
function fn2 (bar: string, baz?: number) {
    // do stuff
}



fn2('foo', 3); // works
fn2('foo'); // works

fn2();
// Compile time error: Expected 1-2 arguments, but got 0
// An argument for 'bar' was not provided.


fn1('foo', 3); // works
fn1('foo'); // works

fn1();
// Compile time error: Expected 1-2 arguments, but got 0
// An argument for 'bar' was not provided.