Type Conversion and Coercion

Type conversion changes a value from one type to another. Explicit conversion is usually clearer than relying on implicit coercion.

const input = "42";
const count = Number(input);
console.log(count + 8); // 50

console.log(String(25)); // "25"
console.log(Boolean("hello")); // true
console.log(Boolean("")); // false

A conversion may fail to produce the value you expect. Number("hello") gives NaN (Not-a-Number), so validate numeric input with Number.isNaN or Number.isFinite after conversion.

Implicit coercion#

The + operator can concatenate strings or add numbers. If either operand becomes a string, the result may be concatenation:

console.log("5" + 1);  // "51"
console.log("5" - 1);  // 4
console.log(2 * "3");  // 6

These rules can surprise readers. Convert values at application boundaries, such as when receiving form input or parsing query parameters, and validate them before calculations.

Equality#

=== compares without coercing operand types first and is the preferred default. == applies coercion rules, which are sometimes useful but often confusing to beginners.

console.log(5 === "5"); // false
console.log(5 == "5");  // true

Practice#

Read a string from a hypothetical form, convert it to a number, reject empty or invalid input, and only then calculate a total.