I have written builder design pattern implementation using inner static class in typeScript.

export class BioData {
        private name: string | undefined;
        private age: number | undefined;
        private section: string | undefined;
    
        static Builder = class {
            protected name: string | undefined;
            protected age: number | undefined;
            protected section: string | undefined;
    
            setName(name: string): this {
                this.name = name;
                return this;
            }
    
            setAge(age: number): this {
                this.age = age;
                return this;
            }

            setSection(section: string): this {
                this.section = section;
                return this;
            }
    
            populate(instance: BioData): void {
                instance.setName(this.name);
                instance.setAge(this.age);
                instance.setSection(this.section);
            }
    
            build(): BioData {
                const instance = new BioData();
                this.populate(instance);
                return instance;
            }
        };
    
        constructor(public builder?: typeof BioData.Builder) {}
    
        setName(name: string | undefined) {
            this.name = name;
        }
    
        setAge(age: number | undefined) {
            this.age = age;
        }

        setSection(section: string | undefined) {
            this.section = section;
       }
    }

when i'm trying to create the object in below way.

const bioDataBuilder = new BioData.Builder();
const bioData: BioData = bioDataBuilder.setName("Shivanshu").setAge(15).secSection("A").build(); 

here the type returned by bioDataBuilder.setName("Shivanshu") is anonymous type(No name). but it should be BioData.Builder. So when i'm trying to call setAge(15) method on object returned by setName("Shivanshu"), this is calling setMethod of BioData class. which is actually wrong.

Can some one please help why the typeReturned by setName("Shivanshu") is anonymous type, although i'm returning this from setName method defined in Inner Static class. How can i make it to returning BioData.Builder.

Thanks.