Validator.js

class Validator {
  constructor(data, rules) {
    this.data = data
    this.rules = rules
    this.errors = {}
  }

  get validations() {
    return {
      required(field) {
        if (!this.data[field]) {
          return `The ${field} is required.`
        }
      },
      string(field) {
        if (typeof this.data[field] !== 'string') {
          return `The ${field} must be a string.`
        }
      },
      integer(field) {
        if (!Number.isInteger(this.data[field])) {
          return `The ${field} must be an integer.`
        }
      },
      maxlength(field, max) {
        if (this.data[field].length > max) {
          return `The ${field} may not be greater than ${max} characters.`
        }
      },
      minvalue(field, min) {
        if (this.data[field] < min) {
          return `The ${field} must be at least ${min}.`
        }
      }
    }
  }

  async validate() {
    for (const field in this.rules) {
      const validations = this.rules[field].split('|')

      for (const validation of validations) {
        const splitted = validation.split(':')
        const validationName = splitted[0]
        const params = (splitted[1] && splitted[1].split(',')) || []
        const message = await this.validations[validationName].call(this, field, ...params)

        if (message) {
          this.errors[field] = message
          break
        }
      }
    }

    if (Object.keys(this.errors).length > 0) {
      throw this.errors
    }
  }
}

// Example
const data = {
  name: 'Renan',
  age: 24
}

const rules = {
  name: 'required|string|maxlength:10',
  age: 'required|integer|minvalue:18'
}

const validator = new Validator(data, rules)

validator.validate()
  .then(() => {
    console.log('Passed')
  })
  .catch(err => {
    console.error(err)
  })