Nest js Tutorial Series — Part 5: Pipes

Pipes

A pipe is a class annotated with the @Injectable() decorator. Pipes implement the PipeTransforminterface. You can use pipes to transform the input data into the desired output format and validate the request data (headers, body, query, query params). Nest interposes a pipe just before a method is invoked, and the pipe receives the arguments destined for the method.

Built-in pipes

@nestjs/common package contains ValidationPipe, ParseIntPipe and ParseUUIDPipe.

Just like Angular, every pipe has to provide the transform() method. This method has two parameters:

• value
• metadata

The value is the currently processed argument, while metadatais its metadata. The metadata object has these properties:

These properties describe the currently processed argument.

type

Indicates whether the argument is a body @Body(), query @Query(), param @Param(), or a custom parameter (read more here).

metatype

Provides the meta-type of the argument, for example, String.

data

The string passed to the decorator, for example @Body('string').

PipeTransform interface

Custom Pipes

Let’s create a custom pipe to convert an array of strings to uppercase.

First, create a class UpperCasePipe in app.pipes.ts inside the src/app.pipes.ts. Below is the snippet for UpperCasePipe.

app.pipes.ts

Now use the UpperCasePipe in route controller by, passing the new instance of the UpperCasePipe in @UsePipes decorator as an argument.

app.controller.ts

Restart the nest project by killing the current session in the terminal and run npm start again.

As you can see above the custom pipe works perfectly !!!.

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