Custom QueryBuilder in Typescript ( MERN Stack)

Unlocking the Power of QueryBuilder:

//=> src/app/modules/student/student.service.ts
const getAllDataFromDB = async (query: Record<string, any>) => {
  const studentQuery = new QueryBuilder(
    Student.find()
      .populate('admissionSemester')
      .populate({
        path: 'academicDepartment',
        populate: {
          path: 'academicFaculty',
        },
      }),
    query,
  )
    .search(studentSearchableFields)
    .filter()
    .sort()
    .paginate()
    .fields();

  const result = await studentQuery.modelQuery;
  return result;
}

//==>src/app/builder/QueryBuilder.ts
import { FilterQuery, Query } from 'mongoose';

class QueryBuilder<T> {
  public modelQuery: Query<T[], T>;
  public query: Record<string, unknown>;

  constructor(modelQuery: Query<T[], T>, query: Record<string, unknown>) {
    this.modelQuery = modelQuery;
    this.query = query;
  }

  search(searchableFields: string[]) {
    const searchTerm = this?.query?.searchTerm;
    if (searchTerm) {
      this.modelQuery = this.modelQuery.find({
        $or: searchableFields.map(
          (field) =>
            ({
              [field]: { $regex: searchTerm, $options: 'i' },
            }) as FilterQuery<T>,
        ),
      });
    }

    return this;
  }

  filter() {
    const queryObj = { ...this.query }; // copy

    // Filtering
    const excludeFields = ['searchTerm', 'sort', 'limit', 'page', 'fields'];

    excludeFields.forEach((el) => delete queryObj[el]);

    this.modelQuery = this.modelQuery.find(queryObj as FilterQuery<T>);

    return this;
  }

  sort() {
    const sort =
      (this?.query?.sort as string)?.split(',')?.join(' ') || '-createdAt';
    this.modelQuery = this.modelQuery.sort(sort as string);

    return this;
  }

  paginate() {
    const page = Number(this?.query?.page) || 1;
    const limit = Number(this?.query?.limit) || 10;
    const skip = (page - 1) * limit;

    this.modelQuery = this.modelQuery.skip(skip).limit(limit);

    return this;
  }

  fields() {
    const fields =
      (this?.query?.fields as string)?.split(',')?.join(' ') || '-__v';

    this.modelQuery = this.modelQuery.select(fields);
    return this;
  }
}

export default QueryBuilder;