Exercise 1: Safe Number Input (string → number) + Sum

Problem Statement

Create a function `sum(a, b)` that accepts inputs as string or number. The function should strictly convert to number and return the sum. Return `null` for invalid input. Example: - sum("10", "5") → 15 - sum("10a", 5) → null

Sample Output:

sum("10","5") => 15
sum("10a",5) => null

Solution

const toNum = (v) => {
  const n = Number(v);
  return Number.isFinite(n) ? n : null;
};

const sum = (a, b) => {
  const x = toNum(a), y = toNum(b);
  return x === null || y === null ? null : x + y;
};

Explanation

Overall Goal:

User se input string ya number me aa sakta hai (form inputs hamesha string hote hain).
Safely number me convert karke add karna hai.
Invalid input (jaise "10a") ho to null return karna hai.

Line 1-2: Helper function `toNum`

const toNum = (v) => { → ek helper function jo kisi bhi value ko safely number me convert karega.
const n = Number(v); → Number() constructor use karke conversion try karte hain.
Number("10") → 10 (valid number)
Number("10a") → NaN (Not a Number, invalid)
Number(5) → 5 (already number, same hi return)

Line 3: Validation

return Number.isFinite(n) ? n : null;
Number.isFinite(n) check karta hai:
Agar n ek finite number hai (10, 5.5, -3) → true
Agar n NaN ya Infinity hai → false
Ternary operator ? : → valid ho to n return, warna null.

Line 5-6: Main `sum` function

const sum = (a, b) => { → main function jo dono inputs ko add karega.
const x = toNum(a), y = toNum(b); → dono inputs ko toNum se convert karke x aur y me store.
Comma operator , use kiya hai taaki ek line me dono variables declare kar sakein.

Line 7: Final check aur addition

return x === null || y === null ? null : x + y;
x === null || y === null → agar koi bhi input invalid ho (null aaya ho)
Agar koi invalid hai → null return (safe failure).
Agar dono valid hain → x + y return (normal addition).

Real world:

Form inputs hamesha string me aate hain (document.getElementById("input").value).
User "10a" type kar sakta hai, isliye validation zaroori hai.
E-commerce carts, calculators, age verification me ye pattern daily use hota hai.

Related Exercises

Exercise 2: Price After Discount (round to 2 decimals)Exercise 3: Template String: User Greeting

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