Here is my case:

/**
 * Add types to the `add` function, so it:
 * - accepts `a` parameter of type string or number
 * - accepts `b` parameter of type string or number
 * - returns type number when both `a` and `b` are of type number
 * - returns type string when at least one param is of type string
 * 
 * ⚠️ There is no need to modyfy the body of the function.
 */

function add(a,b)  {
    if (typeof a === 'number' && typeof b === 'number') {
        return a   b;
    }

    return String(a)   String(b);
}

////////////////////////////////////////////////////////////////////////////////
////////////////////   TEST CASES - look but don't modify   ////////////////////
////////////////////////////////////////////////////////////////////////////////
const sumA = add(1, 2);
const sumB = add(1, '2');
const sumC = add('1', 2);
const sumD = add('1', '2');

type cases = [
    Expect<Equal<typeof sumA, number>>,
    Expect<Equal<typeof sumB, string>>,
    Expect<Equal<typeof sumC, string>>,
    Expect<Equal<typeof sumD, string>>,
]

type Expect<T extends true> = T
type Equal<X, Y> =
  (<T>() => T extends X ? 1 : 2) extends
  (<T>() => T extends Y ? 1 : 2) ? true : false

No matter what I do (conditional return, type aliases) it does not allow me to pass through Expect cases.

Does anyone have an idea how to solve it?

Playground

CodePudding user response：

Overloads will help here:

function add(a: number, b: number): number
function add(a: string | number, b: string | number): string
function add(a: string | number, b: string | number): string | number {
    if (typeof a === 'number' && typeof b === 'number') {
        return a   b;
    }

    return String(a)   String(b);
}

Playground

CodePudding user response：

You can make use of Overloading feature of JavaScript here. Add overloaded definitions of the given function as follows

function add(a: number, b: number) : number
function add(a: string | number, b: string | number) : string
function add(a: string | number,b: string |number) : string | number {
        if (typeof a === 'number' && typeof b === 'number') {
            return a   b;
        }
    return String(a)   String(b);
    }