为什么 TypeScript 编译器在 function 返回 undefined 而函数的返回类型为数字时不报错？

Question

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;
}

Answer 1

add "strict":true under "compilerOptions" in tsconfig.json file.

Answer 2

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

Answer 3

as @SM mentioned above it is caused by strict flag indeed it is caused by strictNullChecks flag.

when I set

"strict": true,

or

"strictNullChecks": true,

TSC gives error as what I expect