Type Coercion: ToPrimitive, ToNumber, ToBoolean

When JavaScript encounters an object where it expects a primitive — during +, -, comparisons, or template literals — it runs an internal algorithm called ToPrimitive. This algorithm decides how to convert any object into a primitive value.

ToPrimitive takes a hint: 'number', 'string', or 'default' (which behaves like 'number' for most built-in objects). The hint determines the ORDER of method calls:

number hint (arithmetic, <, >): call valueOf() first → if primitive, use it → else call toString() → if primitive, use it → else TypeError

string hint (template literals, String()): call toString() first → if primitive, use it → else call valueOf() → if primitive, use it → else TypeError

const obj = {
  valueOf()  { return 42; },
  toString() { return 'hello'; }
};

// 'number' hint: valueOf() first
console.log(obj + 0);          // 42 — valueOf() → 42
console.log(obj - 0);          // 42 — arithmetic: number hint
console.log(obj * 2);          // 84 — arithmetic: number hint

// 'string' hint: toString() first
console.log(`${obj}`);         // 'hello' — template: string hint
console.log(String(obj));      // 'hello' — String(): string hint

// 'default' hint (behaves like number for most objects)
console.log(obj == 42);        // true — default hint → valueOf → 42

// If valueOf returns non-primitive:
const obj2 = {
  valueOf()  { return {}; },   // returns object — not a primitive
  toString() { return 'text'; }
};
console.log(obj2 + '');        // 'text' — valueOf fails, falls to toString