Type Conversion

Values from forms and prompt() are always strings. Convert them before doing maths.

Number("42");      // 42
Number("abc");     // NaN (Not a Number)
parseInt("42px");  // 42
parseFloat("3.5"); // 3.5
String(99);        // "99"
Boolean(0);        // false
Boolean("hi");     // true

Truthy and falsy

These values count as false in an if: false, 0, "", null, undefined, NaN. Everything else is truthy, including "0", [] and {}.