Why TypeScript compiler does not give error when function returns undefined while function's return type is number?

Issue

Why TSC does not give error when function returns null or undefined while function's return type is number.

//gives error
//Error : A function whose declared type is neither 'void' nor 'any' must return a value.ts(2355)
function add1(a: number, b: number): number {}


// no error
function add2(a: number, b: number): number {
  return undefined;
}

// no error
function add3(a: number, b: number): number {
  return null;
}

Solution

You get no error when Type Checking compiler option strictNullChecks is off. Enabling the option will result in Typescript displaying the error for the functions add2 and add3.

You can read more about it: strictnullchecks-off and strictnullchecks-on

Answered By - Siddhant