a.ts

const assert = (cond) => { if (!cond) throw new Error('assertion error') }; 

type Sum = (a: any, b: any) => number;

const addWithNumbers: Sum = (a, b) => a + b;

const numbers = {
    one: 1,
    two: 2,
    three: 3
};

const addWithStrings: Sum = (a, b) => numbers[a] + numbers[b];

console.log(addWithNumbers(10, 20));
console.log(addWithStrings('one', 'three'));

function perform(add: Sum) {
    const result = add(120, 3); 
    console.log(result);
    assert(result === 123);    
}


perform(addWithNumbers);
console.log("with strings");
perform(addWithStrings);


//--------------------------------------------------

assert(addWithStrings('one', 'three') === 4);
assert(addWithNumbers(10, 20) === 30);