Utilizing the NestJS REPL atmosphere

Learn-Eval-Print Loop, higher referred to as REPL, is an interactive pc atmosphere that reads person inputs and evaluates and shows the outcome to the person. A REPL atmosphere is a helpful instrument for rapidly exploring the options and functionalities obtainable in particular system environments and programming languages.

On this article, you’ll discover ways to use the NestJS REPL atmosphere in your initiatives. We’ll moreover spotlight how leveraging the built-in Node REPL atmosphere can take advantage of the NestJS REPL atmosphere.

Leap forward:

Temporary overview of REPL and NestJS

A easy instance of a REPL atmosphere can be the console discovered within the developer instrument of most fashionable internet browsers.

In an identical method, the Node runtime atmosphere additionally has a built-in REPL atmosphere, which you need to use to execute easy JavaScript code interactively.

Just like the environments talked about above, NestJS additionally has a REPL atmosphere that you need to use to examine your software’s dependency graph and invoke strategies in your suppliers and controllers from the terminal.

It’s value noting that the NestJS REPL atmosphere shipped lately with NestJS model 9 — as such, it’s a comparatively new addition to NestJS.

How you can arrange a NestJS REPL atmosphere

To get began, you will need to first arrange and configure the NestJS REPL atmosphere earlier than operating your NestJS software in REPL mode. You probably have a NestJS venture, comply with the steps beneath to configure the NestJS REPL atmosphere. For this, we assume you’re operating a venture created utilizing the Nest CLI and with the boilerplate information intact.

Createrepl.ts file

Create a repl.ts file in the identical listing containing the predominant.ts file. After that, you may copy and paste the next code into it:

js
import { repl } from '@nestjs/core';
import { AppModule } from './app.module';

const bootstrap = async () => {
  await repl(AppModule);
};

bootstrap();

Within the instance above, we declared an asynchronous operate for bootstrapping the NestJS REPL atmosphere.

Launch the NestJS REPL atmosphere

Run the command beneath on the terminal to launch the NestJS REPL atmosphere.

sh
npm run begin -- --entryFile repl

As a substitute of operating the above command every time you need to launch the REPL atmosphere in your venture, it’s handy so as to add a brand new script to your package deal.json file. Everytime you need to open the REPL atmosphere, execute the command npm run begin:repl on the terminal.

json
{
 ...
 "scripts":{
    "begin:repl": "npm run begin -- --entryFile repl"
 }
 ...
}

The repl choice you go to the command above returns an occasion of the Node REPL server object. Due to this fact, all of the functionalities within the Node REPL atmosphere can be found within the NestJS REPL atmosphere.

NestJS REPL atmosphere native capabilities

The NestJS REPL atmosphere comes with a number of native capabilities. This part will discover a number of the predominant options of the NestJS REPL atmosphere.

How you can use the built-in NestJS REPL assist utility

The NestJS REPL atmosphere has a built-in utility you need to use to get assistance on native capabilities within the REPL atmosphere. Use the assist operate in case you are within the interactive REPL mode, as proven right here:

sh
assist()

Invoking the assist operate as within the code above in an interactive REPL session will print the obtainable REPL capabilities and a one-line abstract of what every REPL operate does. A few of these built-in capabilities embrace: get, debug, resolve, choose, and strategies.

Equally, you may also use .assist on any of the aforementioned capabilities to get assistance on a particular REPL operate. For instance, the get.assist command will print a one-line description of the get operate, the parameters it takes, and its return sort.

How you can print registered modules

Modules are the really useful means of writing NestJS functions. They assist in organizing the parts in your software — every NestJS software will need to have no less than a root module, however a typical real-world NestJS software often has a number of modules.

The debug operate turns out to be useful if you wish to print a listing of all of the registered modules with their controllers and suppliers. Invoke the debug operate in NestJS REPL like so:

sh
debug() 

If you wish to print the parts of a particular module, go the module as an argument:

sh
debug(AppModule)

As well as, you may also use the debug.assist command to entry the built-in REPL documentation. It returns a one-line description of the debug operate and its operate signature, like so:

How you can view public strategies in controllers and suppliers

Controllers and suppliers are among the many core constructing blocks of NestJS functions. In a typical NestJS software, controllers deal with HTTP requests and responses whereas delegating extra advanced duties to suppliers.

You should utilize the strategies operate to view the general public strategies in your software’s controllers and suppliers. To print the strategies in a particular controller or supplier, go the identify of the controller or supplier as an argument to the strategies operate like this:

sh
strategies(QuotesController)

As well as, you may also use the strategies.assist command to entry the built-in documentation concerning the strategies operate. Run the strategies.assist command on the terminal to print a one-line abstract of the documentation and its operate signature.

How you can retrieve cases of controllers or injectables

The first advantage of the NestJS REPL is interacting with the NestJS software from the command line.

The get operate turns out to be useful if you wish to entry cases of your software’s controllers and injectables, and it’s also helpful for testing the strategies in your controllers and providers from the command line — remember that you need to use get and $ interchangeably.

The instance beneath reveals how one can retrieve an occasion of a controller.

sh
get(QuotesController).getQuotes()

You too can entry the controller or service occasion and put it aside in a variable earlier than utilizing it, like this:

sh 
const quotesController = get(QuotesController)
quotesController.getQuotes()

Like the opposite capabilities highlighted above, you need to use the get.assist or $.assist command to print the built-in get documentation on the terminal.

How you can resolve transient cases of controllers or injectables

As identified above, you need to use the get operate to retrieve cases of controllers and injectables.

Nevertheless, utilizing get with scoped or transient suppliers will throw an error. Due to this, it’s good to use the resolve operate to resolve transient cases of injectables or controllers. Its utilization is much like the opposite capabilities talked about above.

You possibly can go the request-scoped injectable or controller to the resolve operate like this:

sh
const quotesController = await resolve(QuotesController)
quotesController.getQuotes()

Equally, use the resolve.assist command to get a one-line description of the resolve operate and its argument and return sort.

How you can navigate the module tree

The choose operate is available in helpful if you wish to navigate the module tree after deciding on a particular module. Like earlier capabilities, choose takes the module you need to choose as an argument.

sh
choose(AppModule).get(QuotesController).getQuotes()

Equally, use the choose.assist command to entry the built-in documentation concerning the choose operate.

How you can use the Node REPL atmosphere

Launching the NestJS REPL atmosphere returns the Node REPL server object. Due to this fact, you may entry all of the options of the Node REPL atmosphere from throughout the NestJS REPL. In different phrases, you may get essentially the most out of the NestJS REPL through the use of it alongside the built-in Node REPL.

On this part, we are going to have a look at some options of the Node REPL atmosphere.

How you can save evaluated instructions to a file

The Node REPL, which you’ll be able to entry from throughout the NestJS REPL atmosphere, supplies performance for saving evaluated instructions in a particular REPL session to a file.

You should utilize the .save command with the identify of the file, like so:

sh
.save check.js

The above command will create the check.js file and saves the instructions evaluated within the present REPL session.

How you can load JavaScript file into present REPL session

It can save you evaluated instructions to a file. Likewise, you may load the contents of a file to the present REPL session utilizing the .load command. You have to specify the file identify, as within the instance beneath.

sh
.load check.js

How you can enter editor mode

Within the REPL mode, you may execute just one full assertion at a time. To enter a number of traces of code earlier than execution, launch the editor mode utilizing the .editor command.

After that, you may enter multi-line code earlier than execution. You possibly can exit the editor mode utilizing the CTRL+D key mixture or cancel it utilizing the CTRL+C key mixture.

The particular underscore variable

The Node REPL atmosphere has the _ variable, which you need to use to entry the results of the final operation from the NestJS REPL atmosphere. This can be a helpful variable if you wish to use the results of the earlier operation.

The above options are on no account exhaustive. Try the Node documentation for added options of the Node REPL atmosphere that may be helpful within the NestJS REPL atmosphere.

Conclusion

The NestJS REPL atmosphere shipped with NestJS 9 and is a comparatively new addition to NestJS.

Like most REPL environments, you need to use it to work together together with your NestJS software from the command line. It’s notable in that its makes use of embrace inspecting your software’s dependency graph and invoking strategies in your suppliers and controllers from the terminal.

As we noticed beforehand, the NestJS REPL atmosphere has a number of native capabilities. A few of these capabilities embrace: get, debug, resolve, and choose. To get essentially the most out of it, use the NestJS REPL atmosphere with the built-in Node REPL atmosphere.

Let me know within the feedback part beneath if there’s something I’ve missed or your personal experiences utilizing the NestJS REPL atmosphere!