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Parsing Requests

This is an action in the default HTTP sequence, it parses arguments from an incoming request and uses them as inputs to invoke the corresponding controller method.

This action contains 3 steps:

  • Parses arguments from request query, body, path and header according to the operation’s OpenAPI specification.
  • Coerces parameters from string to its corresponding JavaScript run-time type.
  • Performs validation on the parameters and body data.

Parsing Raw Data

The code below defines a typical endpoint by decorating a controller method with rest decorators.

class TodoController {
  constructor(@repository(TodoRepository) protected todoRepo: TodoRepository) {}

  async replaceTodo(
    @param.path.number('id') id: number,
    @requestBody() todo: Todo,
  ): Promise<boolean> {
    return this.todoRepo.replaceById(id, todo);

An OpenAPI operation specification will be generated in-memory to describe it, and raw data is parsed from request according to the specification. In the example above, the first parameter is from source path, so its value will be parsed from a request’s path.


The parameters parsed from path, header, and query of a http request are always in the string format when using the http module in Node.js to handle requests. Therefore when invoking a controller function, a parameter need to be converted to its corresponding JavaScript runtime type, which is inferred from its parameter specification.

For example, the operation replaceTodo in section parsing raw data takes in a number id as the first input. Without coercion,id would have to be manually cast into the number type before it can be used as seen below:

async replaceTodo(
  @param.path.number('id') id: number,
  @requestBody() todo: Todo,
): Promise<boolean> {
  // NO need to do the "string to number" convertion now,
  // coercion automatically handles it for you.
  id = +id;
  return this.todoRepo.replaceById(id, todo);

Object values

OpenAPI specification describes several ways how to encode object values into a string, see Style Values and Style Examples.

At the moment, LoopBack supports both url-encoded and exploded values for json query parameters. Please note that this style does not preserve the encoding of primitive types like numbers and booleans. They are always parsed as strings.

To filter results from the GET /todo-list endpoint in the todo-list example with a relation, { “include”: [ { “relation”: “todo” } ] }, the following url-encoded query parameter can be used,


As an extension to the url-encoded style, LoopBack also supports queries with exploded values for json query parameters.

For example:

GET /todos?filter[where][completed]=false
// filter={where: {completed: 'false'}}


Validations are applied on the parameters and the request body data. They also use OpenAPI specification as the reference to infer the validation rules.


We have the data type safety check for the parameters parsed from header, path, and query. For example, if a parameter should be an integer, then a number with decimal like “1.23” would be rejected.

You can specify a parameter’s type by calling shortcut decorators of @param like @param.query.integer(). A list of available shortcuts can be found in the API Docs. Check out the section on parameter decorators for instructions on how to decorate the controller parameter.

Here are our default validation rules for each type:

  • number: validated by isNaN(Number(data)).
  • integer: validated by Number.isInteger(data).
  • long: validated by Number.isSafeInteger(data).
  • date-time: should be a valid date-time defined in RFC3339.
  • date: should be a valid full-date defined in RFC3339.
  • boolean: after converted to all upper case, should be one of the following values: TRUE, 1, FALSE or 0.
  • object: should be a plain data object, not an array.

Request Body

The data from request body is validated against its OpenAPI schema specification. We use AJV module to perform the validation, which validates data with a JSON schema generated from the OpenAPI schema specification.

Take again the operation replaceTodo for instance:

import {Todo} from './models';

// class definition
  async replaceTodo(
    @param.path.number('id') id: number,
    @requestBody() todo: Todo,
  ): Promise<boolean> {
    return this.todoRepo.replaceById(id, todo);

The request body specification is defined by applying @requestBody() to argument todo, and the schema specification inside it is inferred from its type Todo. The type is exported from a Todo model.

When the PUT method on the /todo/{id} gets called, the todo instance from the request body will be validated with a well defined specification.

Validation of model objects is heavily dependent on its OpenAPI Schema defined in/by the @requestBody decorator. Please refer to the documentation on @requestBody decorator to get a comprehensive idea of defining custom validation rules for your models.

You can also specify the JSON schema validation rules in the model property decorator. The rules are added in a field called jsonSchema, like:

class Product extends Entity {
    name: 'name',
    description: "The product's common name.",
    type: 'string',
    // Specify the JSON validation rules here
    jsonSchema: {
      maxLength: 30,
      minLength: 10,
  public name: string;

A full list of validation keywords could be found in the documentation of AJV validation keywords.

Common tasks