REPL in Nestjs

Published at

7/14/2024

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One of the features of Nestjs is Tte REPL (Read-Eval-Print Loop) mode, a powerful tool that allows you to interactively test and execute code within the context of your NestJS application. This is particularly useful for quick experimentation, testing services, or debugging from the terminal. Here’s a step-by-step guide on how to use the REPL mode in NestJS:

create a new file repl.ts

import { existsSync, mkdirSync } from "node:fs";
import { join } from "node:path";

import { Logger } from "@nestjs/common";
import { repl } from "@nestjs/core";

import { AppModule } from "./app.module";

const logger = new Logger("Repl");

async function bootstrap() {
  const replServer = await repl(AppModule);

  // OPTIONAL: sets up persistant history file for repl, 
  const cacheDirectory = join("node_modules", ".cache");

  if (!existsSync(cacheDirectory))
    mkdirSync(cacheDirectory);

  replServer.setupHistory(
    join(cacheDirectory, ".nestjs_repl_history"),
    (error) => {
      if (error)
        logger.error(error);
    },
  );
}
bootstrap();

Then you can run the app in repl mode with :

npm run start -- --entryFile repl

Even better, add a script on your package.json

"start:repl":"npm run start -- --entryFile repl"

It will initiate an interactive server, from which you can easily access your nest app methods by getting them either as

get(AppService).getAllPosts()

$(AppService).getAllPosts()

You can also navigate with the command history with ↕️ arrow keys

dev-resources.site

REPL in Nestjs