Question

What's the output?

function sum(a, b) {
  return a + b;
}

sum(1, '2');

• A: NaN
• B: TypeError
• C: "12"
• D: 3

Answer 1

Answer: C

JavaScript is a dynamically typed language: we don't specify what types certain variables are. Values can automatically be converted into another type without you knowing, which is called implicit type coercion. Coercion is converting from one type into another.

In this example, JavaScript converts the number 1 into a string, in order for the function to make sense and return a value. During the addition of a numeric type (1) and a string type ('2'), the number is treated as a string. We can concatenate strings like "Hello" + "World", so what's happening here is "1" + "2" which returns "12".