math-well-known-symbol-example.js

Math.symbol = {
	ceil: Symbol('Math.symbol.ceil')
};

class ComplexNumber {
	constructor(a, b) {
		this.a = a;
		this.b = b;
	}
	toString() {
		return `${this.a} + ${this.b}i`;
	}
	[Symbol.toPrimitive](hint) {
		if(hint === 'string') return this.toString();
		throw TypeError('Cannot convert a ComplexNumber value to a number.')
	}
	[Math.symbol.ceil]() {
		return new ComplexNumber(
			Math.ceil(this.a),
			Math.ceil(this.b),
		);
	}
}

const complex = ComplexNumber(4.9, 6.3);
complex.toString(); // 4.9 + 6.3i
Math.ceil(complex).toString(); // 5 + 7i