Pipes

A pipe is a class annotated with the Injectable decorator. Pipes should implement the IPipe interface.

Pipes have two typical use cases:

• transformation: transform input data to the desired output
• validation: evaluate input data and if valid, simply pass it through unchanged; otherwise, throw an exception when the data is incorrect

Pipes are called when an Incoming request is handled by the controller route handler and operate on the method's parameters. It takes Request or Response object and transform theses object to the expected value.

Pipe receives the argument where it is placed. This means that each parameter can invoke a list of pipes, which can be different for each parameter.

@Get("/")
getMethod(@UsePipe(MyPipe) value: string, @UsePipe(MyPipe2) @UsePipe(MyPipe3) value2: string) {}

Finally, both transformation and validation must implement a transform() method and return the expected value.

TIP

Pipes run inside an exception zone. It means that when a Pipe throws an exception, it will be handled by the GlobalExceptionHandler . Given the above, it should be clear that when an exception is thrown in a Pipe, no controller method is subsequently executed.

Built-in pipes

Ts.ED comes with the following pipes:

• ParseExpressionPipe ,
• ValidationPipe ,
• DeserializerPipe

These pipes are exported to allow pipes overriding. These decorators are commonly used by BodyParams , QueryParams , etc. In this case, the pipes are added by using UseParam on a parameter.

For example, the use of BodyParams on a parameter calls the UseParams with some options, and UseParams calls also different decorators to add Pipes:

The main idea is, you are able to combine any pipes to reach the expected behavior !

Now let's build a validation pipe from scratch to understand the pipe mechanism.

Initially, we'll have it simply take an input value and immediately return the same value, behaving like an identity function.

import {JsonParameterStore, PipeMethods} from "@tsed/schema";
import {Injectable} from "@tsed/di";

@Injectable()
export class ValidationPipe implements PipeMethods {
  transform(value: any, metadata: JsonParameterStore) {
    return value;
  }
}

TIP

IPipe<T, R> is a generic interface in which T indicates the type of the input value, and R indicates the return type of the transform() method.

Every pipe has to provide the transform() method. This method has two parameters:

• value
• metadata

The value is the currently processed argument (before it is received by the route handling method), while metadata is its metadata. The metadata object has these properties (see also ParamMetadata ):

class ParamMetadata {
  target: Type<any>;
  propertyKey: string | symbol;
  index: number;
  required: boolean;
  paramType: string | ParamTypes;
  expression: string;
  type: Type<any>;
  collectionType: Type<any>;
  pipes: Type<IPipe>[];
  store: Store;
}

These properties describe the current processed argument.

Property	Description
target	The parameter's class
propertyKey	The parameter's method
index	The position of the parameter in the method signature
required	Indicates whether the parameter is required or not
paramType	ParamTypes represents the starting object used by the first pipe
expression	Expression used to get the property from the object injected with paramType
type	Class used to deserialize the plain object
collectionType	Collection type used to deserialize a collection of plain object
store	Store contains extra options collected by the decorators used on the parameter.

WARNING

TypeScript interfaces disappear during transpilation. Thus, if a method parameter's type is declared as an interface instead of a class, the type value will be Object.

Validation use case

The goal of validation use case is to ensure that the input parameter is valid before using it in a method.

Officially, Ts.ED has two way to declare a JsonShema validation:

• With model decorators,
• With UseSchema decorator, it's a decorator Pipe provided by @tsed/ajv package.

We'll take the model declaration to explain the Validation pipe use case. Let's focus on the PersonModel:

import {MinLength, Required} from "@tsed/common";

class PersonModel {
  @MinLength(3)
  @Required()
  firstName: string;

  @MinLength(3)
  @Required()
  lastName: string;
}

PersonModel will generate the following JsonSchema:

{
  "type": "object",
  "properties": {
    "firstName": {
      "type": "string",
      "minLength": 3
    },
    "lastName": {
      "type": "string",
      "minLength": 3
    }
  },
  "required": ["firstName", "lastName"]
}

We want to ensure that any incoming request to the create method contains a valid body. So we have to validate the two members of the PersonModel object, used as type parameter:

import {Post} from "@tsed/schema";
import {Controller} from "@tsed/di";
import {BodyParams} from "./body-params";
import {PersonModel} from "../models/PersonModel";

@Controller("/persons")
export class PersonsController {
  @Post("/")
  save(@BodyParams() person: PersonModel) {
    return person;
  }
}

By using a pipe, we are able to handle the parameter, get its schema and use a validation library (here AJV) and throw an exception when the payload is not valid.

import {ValidationError} from "@tsed/platform-params";
import {getJsonSchema, JsonParameterStore, PipeMethods} from "@tsed/schema";
import {Injectable} from "@tsed/di";
import * as Ajv from "ajv";

@Injectable()
export class AjvValidationPipe implements PipeMethods {
  ajv = new Ajv();

  transform(value: any, metadata: JsonParameterStore): any {
    const schema = getJsonSchema(metadata.type);

    if (!this.ajv.validate(schema, value)) {
      throw new ValidationError("Oops something is wrong", this.ajv.errors!);
    }

    return value;
  }
}

The validation pipe is a very specific use case because Ts.ED uses it automatically when a parameter is handled by the routing request. The previous pipe example, in order to work, needs to be registered with the OverrideProvider decorator instead of Injectable .

See more details on the validation page.

Transformation use case

Validation isn't the sole use case for Pipes. At the beginning of this chapter, we mentioned that a pipe can also transform the input data to the desired output. This is possible because the value returned from the transform function completely overrides the previous value of the argument.

When is this useful? Consider that sometimes the data passed from the client needs to undergo some changes - for example converting plain object javascript to class - before it can be properly handled by the route handler method. Furthermore, some required data fields may be missing, and we would like to apply default values.

Transformer pipes can perform these functions by interposing a processing function between the client request and the request handler.

import {ValidationError} from "@tsed/platform-params";
import {JsonParameterStore, PipeMethods} from "@tsed/schema";
import {Injectable} from "@tsed/di";

@Injectable()
export class ParseIntPipe implements PipeMethods<string, number> {
  transform(value: string, metadata: JsonParameterStore): number {
    const val = parseInt(value, 10);
    if (isNaN(val)) {
      throw new ValidationError("Value must an integer or a parsable integer");
    }

    return val;
  }
}

We can simply tie this pipe to the selected param as shown below:

import {RawPathParams, UsePipe} from "@tsed/platform-params";
import {Get} from "@tsed/schema";
import {Controller, Inject} from "@tsed/di";
import {ParseIntPipe} from "../pipes/ParseIntPipe";
import {PersonsService} from "../services/PersonsService";

@Controller("/persons")
export class PersonsController {
  @Inject()
  private personsService: PersonsService;

  @Get(":id")
  async findOne(
    @RawPathParams("id")
    @UsePipe(ParseIntPipe)
    id: number
  ) {
    return this.personsService.findOne(id);
  }
}

TIP

On the previous example, we use RawPathParams to get the raw value, without transformation or validation from existing Ts.ED Pipe.

Async transformation use case

Pipe transformation also supports async and promise as a returned value. This is useful when you have to get data from database based on an input data like an ID.

Given this PersonModel:

import {MinLength, Required} from "@tsed/common";
import {Property} from "./property";

class PersonModel {
  @Property()
  id: string;

  @MinLength(3)
  @Required()
  firstName: string;

  @MinLength(3)
  @Required()
  lastName: string;
}

We can implement the following pipe to get Person data from database:

import {JsonParameterStore, PipeMethods} from "@tsed/schema";
import {Inject, Injectable} from "@tsed/di";
import {NotFound} from "@tsed/exceptions";
import {PersonModel} from "../models/PersonModel";
import {PersonsService} from "../models/PersonsService";

@Injectable()
export class PersonPipe implements PipeMethods<string, Promise<PersonModel>> {
  @Inject()
  personsService: PersonsService;

  async transform(id: string, metadata: JsonParameterStore): Promise<PersonModel> {
    const person = await this.personsService.findOne(id);

    if (!person) {
      throw new NotFound("Person not found");
    }

    return person;
  }
}

Then, we can use this pipe on a parameter with UsePipe :

import {RawPathParams, UsePipe} from "@tsed/platform-params";
import {Put} from "@tsed/schema";
import {Controller} from "@tsed/di";
import {PersonModel} from "../models/PersonModel";
import {PersonPipe} from "../services/PersonPipe";

@Controller("/persons")
export class PersonsController {
  @Put("/:id")
  async update(
    @RawPathParams("id")
    @UsePipe(PersonPipe)
    person: PersonModel
  ) {
    // do something

    return person;
  }
}

Custom pipe decorator

In the previous section, we show you how to use a Pipe on a parameter:

import {RawPathParams, UsePipe} from "@tsed/platform-params";
import {Put} from "@tsed/schema";
import {Controller} from "@tsed/di";
import {PersonModel} from "../models/PersonModel";
import {PersonPipe} from "../services/PersonPipe";

@Controller("/persons")
export class PersonsController {
  @Put("/:id")
  async update(
    @RawPathParams("id")
    @UsePipe(PersonPipe)
    person: PersonModel
  ) {
    // do something

    return person;
  }
}

In this example, our pipe need to be called with RawPathParams two work properly, because our pipe return an instance of PersonModel. PathParams call automatically the DeserializerPipe and it's not what we want. This is why we using RawPathParams .

To avoid future mistakes, it could be a good idea to summarize these two decorators in one as following:

import {RawPathParams, UsePipe} from "@tsed/platform-params";
import {useDecorators} from "@tsed/core";
import {PersonPipe} from "../services/PersonPipe";

export function UsePersonParam(expression: string): ParameterDecorator {
  return useDecorators(RawPathParams(expression), UsePipe(PersonPipe));
}

Now, we can use our custom decorator on parameter:

import {Controller, Put, RawPathParams, UsePipe} from "@tsed/common";
import {PersonModel} from "../models/PersonModel";
import {PersonPipe} from "../services/PersonPipe";

@Controller("/persons")
export class PersonsController {
  @Put("/:id")
  async update(@UsePersonParam("id") person: PersonModel) {
    // do something

    return person;
  }
}

Get options from decorator

Sometimes it might be useful to forward options from a decorator used on parameters to the registered Pipe.

Let's focus on our previous decorator example, by adding extra parameter options:

import {RawPathParams, UsePipe} from "@tsed/platform-params";
import {useDecorators} from "@tsed/core";
import {PersonPipe} from "../services/PersonPipe";

export interface IUsePersonParamOptions {
  optional?: boolean;
}

export function UsePersonParam(expression: string, options: IUsePersonParamOptions = {}): ParameterDecorator {
  return useDecorators(
    RawPathParams(expression),
    UsePipe(PersonPipe, options) // UsePipe accept second parameter to store your options
  );
}

Now, we can retrieve the options by using the metadata.store:

import {Inject, Injectable} from "@tsed/di";
import {NotFound} from "@tsed/exceptions";
import {JsonParameterStore, PipeMethods} from "@tsed/schema";
import {IUsePersonParamOptions} from "../decorators/UsePersonParam";
import {PersonModel} from "../models/PersonModel";
import {PersonsService} from "../models/PersonsService";

@Injectable()
export class PersonPipe implements PipeMethods<string, Promise<PersonModel>> {
  @Inject()
  personsService: PersonsService;

  async transform(id: string, metadata: JsonParameterStore): Promise<PersonModel> {
    const person = await this.personsService.findOne(id);
    const options = metadata.store.get<IUsePersonParamOptions>(PersonPipe);

    if (!person && options.optional) {
      throw new NotFound("Person not found");
    }

    return person;
  }
}

And finally, we can use our new decorator on a parameter:

import {Controller, Put, RawPathParams, UsePipe} from "@tsed/common";
import {PersonModel} from "../models/PersonModel";
import {PersonPipe} from "../services/PersonPipe";

@Controller("/persons")
export class PersonsController {
  @Put("/:id")
  async update(@UsePersonParam("id", {optional: true}) person: PersonModel) {
    // do something

    return person;
  }
}