Hint
Returning this from methods enables fluent interfaces and method chaining.
class Builder {
constructor() { this.parts = []; }
add(part) { this.parts.push(part); return this; }
build() { return this.parts.join(' + '); }
}
console.log(new Builder().add('A').add('B').add('C').build());A + B + C
Explanation: add() returns this, enabling chaining. build() collects all parts.
Key Insight: Returning this from methods enables fluent interfaces and method chaining.