Node.js v24.0.1 documentation

JavaScript expressions#

The default evaluator supports direct evaluation of JavaScript expressions:

> 1 + 1
2
> const m = 2
undefined
> m + 1
3 copy

Unless otherwise scoped within blocks or functions, variables declared either implicitly or using the const, let, or var keywords are declared at the global scope.

Global and local scope#

The default evaluator provides access to any variables that exist in the global scope. It is possible to expose a variable to the REPL explicitly by assigning it to the context object associated with each REPLServer:

import repl from 'node:repl';
const msg = 'message';

repl.start('> ').context.m = msg;const repl = require('node:repl');
const msg = 'message';

repl.start('> ').context.m = msg;copy

Properties in the context object appear as local within the REPL:

$ node repl_test.js
> m
'message' copy

Context properties are not read-only by default. To specify read-only globals, context properties must be defined using Object.defineProperty():

import repl from 'node:repl';
const msg = 'message';

const r = repl.start('> ');
Object.defineProperty(r.context, 'm', {
  configurable: false,
  enumerable: true,
  value: msg,
});const repl = require('node:repl');
const msg = 'message';

const r = repl.start('> ');
Object.defineProperty(r.context, 'm', {
  configurable: false,
  enumerable: true,
  value: msg,
});copy

Accessing core Node.js modules#

The default evaluator will automatically load Node.js core modules into the REPL environment when used. For instance, unless otherwise declared as a global or scoped variable, the input fs will be evaluated on-demand as global.fs = require('node:fs').

> fs.createReadStream('./some/file'); copy

Global uncaught exceptions#

The REPL uses the domain module to catch all uncaught exceptions for that REPL session.

This use of the domain module in the REPL has these side effects:

• Uncaught exceptions only emit the 'uncaughtException' event in the standalone REPL. Adding a listener for this event in a REPL within another Node.js program results in ERR_INVALID_REPL_INPUT.

  const r = repl.start();
  
  r.write('process.on("uncaughtException", () => console.log("Foobar"));\n');
  // Output stream includes:
  //   TypeError [ERR_INVALID_REPL_INPUT]: Listeners for `uncaughtException`
  //   cannot be used in the REPL
  
  r.close(); copy

• Trying to use process.setUncaughtExceptionCaptureCallback() throws an ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE error.

Assignment of the _ (underscore) variable#

The default evaluator will, by default, assign the result of the most recently evaluated expression to the special variable _ (underscore). Explicitly setting _ to a value will disable this behavior.

> [ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
Expression assignment to _ now disabled.
4
> 1 + 1
2
> _
4 copy

Similarly, _error will refer to the last seen error, if there was any. Explicitly setting _error to a value will disable this behavior.

> throw new Error('foo');
Uncaught Error: foo
> _error.message
'foo' copy

await keyword#

Support for the await keyword is enabled at the top level.

> await Promise.resolve(123)
123
> await Promise.reject(new Error('REPL await'))
Uncaught Error: REPL await
    at REPL2:1:54
> const timeout = util.promisify(setTimeout);
undefined
> const old = Date.now(); await timeout(1000); console.log(Date.now() - old);
1002
undefined copy

One known limitation of using the await keyword in the REPL is that it will invalidate the lexical scoping of the const keywords.

For example:

> const m = await Promise.resolve(123)
undefined
> m
123
> m = await Promise.resolve(234)
234
// redeclaring the constant does error
> const m = await Promise.resolve(345)
Uncaught SyntaxError: Identifier 'm' has already been declared copy

--no-experimental-repl-await shall disable top-level await in REPL.