app.js

const user = { firstname: 'x', lastname: 'x', email: 'x', password: 'x' }

var schema = {
    type: 'object',
    required: ['firstname', 'lastname', 'email', 'password'],
    allOf: [{
      properties: {
        firstname: { type: 'string', minLength: 2, maxLength: 35 },
        lastname: {type: 'string', minLength: 2, maxLength: 35},
        email: {type: 'string', format: 'email'},
        password: {type: 'string', minLength: 2, maxLength: 35}
      },
      additionalProperties: false
    }],
    errorMessage: {
      properties: {
        firstname: 'test custom message'
      }
    }
  };

var validate = ajv.compile(schema);
  console.log(validate(user)); // false
  console.log(validate.errors); // processed errors


// outputs
[ { keyword: 'minLength',
    dataPath: '.firstname',
    schemaPath: '#/allOf/0/properties/firstname/minLength',
    params: { limit: 2 },
    message: 'should NOT be shorter than 2 characters' },
  { keyword: 'minLength',
    dataPath: '.lastname',
    schemaPath: '#/allOf/0/properties/lastname/minLength',
    params: { limit: 2 },
    message: 'should NOT be shorter than 2 characters' },
  { keyword: 'format',
    dataPath: '.email',
    schemaPath: '#/allOf/0/properties/email/format',
    params: { format: 'email' },
    message: 'should match format "email"' },
  { keyword: 'minLength',
    dataPath: '.password',
    schemaPath: '#/allOf/0/properties/password/minLength',
    params: { limit: 2 },
    message: 'should NOT be shorter than 2 characters' } ]