Nodejs – REPL
The REPL(Read-Eval-Print-Loop) provides an interactive environment to interact with the core Nodejs module.
It’s a handy utility to test small snippets and explore various Nodejs packages from command line.
Starting REPL
The simplest way to start this REPL shell is by using node command as follows:
The other tab shows some of the useful commands available in REPL just for reference.
F:\nodejs\repl>node Welcome to Node.js v12.18.3. Type ".help" for more information. >
> .help .break Sometimes you get stuck, this gets you out .clear Alias for .break .editor Enter editor mode .exit Exit the repl .help Print this help message .load Load JS from a file into the REPL session .save Save all evaluated commands in this REPL session to a file Press ^C to abort current expression, ^D to exit the repl
This internally starts repl.REPLServer. We can also start instances of this server inside our programs by importing the following module.
const repl = require('repl');
As we will see at the end, we can start multiple instances to work on the same Nodejs process. But, to start with lets see some of the useful features we can get out of this REPL servers.
1. Evaluate Commands on REPL
We can use it for evaluating simple single or multi-line commands and functions on Nodejs.
> 5+5
10
> throw new Error("Oops...");
Uncaught Error: Oops...
> _error.message //Shows last seen error message
'Oops...'
>
> function hello(name){
... console.log("Hello "+ name);
... }
undefined
> hello("Nodejs");
Hello Nodejs
undefined
Three dots shows the continuance of the same program in the next line.
REPL also provides autocomplete features while typing Nodejs commands.
2. Explore Global Objects
As shown below, we can explore the global objects in the Nodejs by using global. + [TAB] command.
> global. global.__defineGetter__ global.__defineSetter__ global.__lookupGetter__ global.__lookupSetter__ global.__proto__ global.hasOwnProperty global.isPrototypeOf global.propertyIsEnumerable global.toLocaleString global.toString global.valueOf global.constructor global.Array global.ArrayBuffer global.Atomics global.BigInt global.BigInt64Array global.BigUint64Array global.Boolean global.Buffer global.DataView global.Date global.Error global.EvalError global.Float32Array global.Float64Array global.Function global.GLOBAL global.Infinity global.Int16Array global.Int32Array global.Int8Array global.Intl global.JSON global.Map global.Math global.NaN global.Number global.Object global.Promise global.Proxy global.RangeError global.ReferenceError global.Reflect global.RegExp global.Set global.SharedArrayBuffer global.String global.Symbol global.SyntaxError global.TextDecoder global.TextEncoder global.TypeError global.URIError global.URL global.URLSearchParams global.Uint16Array global.Uint32Array global.Uint8Array global.Uint8ClampedArray global.WeakMap global.WeakSet global.WebAssembly global._ global._error global.assert global.async_hooks global.buffer global.child_process global.clearImmediate global.clearInterval global.clearTimeout global.cluster global.console global.crypto global.decodeURI global.decodeURIComponent global.dgram global.dns global.domain global.encodeURI global.encodeURIComponent global.escape global.eval global.events global.fs global.global global.globalThis global.http global.http2 global.https global.inspector global.isFinite global.isNaN global.module global.net global.os global.parseFloat global.parseInt global.path global.perf_hooks global.process global.punycode global.querystring global.queueMicrotask global.readline global.repl global.require global.root global.setImmediate global.setInterval global.setTimeout global.stream global.string_decoder global.tls global.trace_events global.tty global.undefined global.unescape global.url global.util global.v8 global.vm global.worker_threads global.zlib
3. Explore Packages
We can explore the constants and functions available in a Nodejs module as shown for file system(fs) module.
> fs. fs.__defineGetter__ fs.__defineSetter__ fs.__lookupGetter__ fs.__lookupSetter__ fs.__proto__ fs.constructor fs.hasOwnProperty fs.isPrototypeOf fs.propertyIsEnumerable fs.toLocaleString fs.toString fs.valueOf fs.Dir fs.Dirent fs.F_OK fs.FileReadStream fs.FileWriteStream fs.R_OK fs.ReadStream fs.Stats fs.W_OK fs.WriteStream fs.X_OK fs._toUnixTimestamp fs.access fs.accessSync fs.appendFile fs.appendFileSync fs.chmod fs.chmodSync fs.chown fs.chownSync fs.close fs.closeSync fs.constants fs.copyFile fs.copyFileSync fs.createReadStream fs.createWriteStream fs.exists fs.existsSync fs.fchmod fs.fchmodSync fs.fchown fs.fchownSync fs.fdatasync fs.fdatasyncSync fs.fstat fs.fstatSync fs.fsync fs.fsyncSync fs.ftruncate fs.ftruncateSync fs.futimes fs.futimesSync fs.lchmod fs.lchmodSync fs.lchown fs.lchownSync fs.link fs.linkSync fs.lstat fs.lstatSync fs.mkdir fs.mkdirSync fs.mkdtemp fs.mkdtempSync fs.open fs.openSync fs.opendir fs.opendirSync fs.promises fs.read fs.readFile fs.readFileSync fs.readSync fs.readdir fs.readdirSync fs.readlink fs.readlinkSync fs.readv fs.readvSync fs.realpath fs.realpathSync fs.rename fs.renameSync fs.rmdir fs.rmdirSync fs.stat fs.statSync fs.symlink fs.symlinkSync fs.truncate fs.truncateSync fs.unlink fs.unlinkSync fs.unwatchFile fs.utimes fs.utimesSync fs.watch fs.watchFile fs.write fs.writeFile fs.writeFileSync fs.writeSync fs.writev fs.writevSync
4. Customize REPL
As mentioned above, we can also create the repl server instance using the repl module. Here, is an example of customizing the repl console :
- We have customized the prompt
- We have initialized the repl context with a variable and
- Finally, we have also added a new function to it.
const repl = require('repl');
function initializeContext(context) {
context.desc = 'My REPL Console';
}
const replServer = repl.start({ prompt: 'My REPL ==> ' });
initializeContext(replServer.context);
replServer.defineCommand('bye', () => {
console.log('bye bye!');
replServer.close();
});
F:\nodejs\repl>node create-repl-server-02.js My REPL ==> desc 'My REPL Console' My REPL ==> My REPL ==> 2+5 7 My REPL ==> .bye bye bye!
Spectrums 