Simple and easy stdio capture for Node.js
Last updated 3 years ago by zackehh .
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capture-console is a small Node.js library built to help when capturing log output via process.stdout and process.stderr. The main use case is unit testing (which is why I built it), but there's no reason it can't be used in production code paths.


capture-console lives on npm, so just install it via the command line and you're good to go.

$ npm install --save-dev capture-console


There are a whole bunch of ways to use capture-console, mainly due to scoping, with the two easiest defined below. Depending on your use case you might be pushed more towards one than the other, but in general you can just choose your preference.

Scoped Captures

The easiest way to use capture-console is with scoping; this is when the output of a provided function is captured.

Note that this form assumes synchronous execution - async stuff will require manual hookups (below).

var capcon = require('capture-console');

var stderr = capcon.captureStderr(function scope() {
  // whatever is done in here has stderr captured,
  // the return value is a string containing stderr

var stdout = capcon.captureStdout(function scope() {
  // whatever is done in here has stdout captured,
  // the return value is a string containing stdout

var stdio = capcon.captureStdio(function scope() {
  // whatever is done in here has both stdout and stderr captured,
  // the return value is an object with 'stderr' and 'stdout' keys

Manual Captures

There are also ways to manually stop and start a capture context, by passing a process stream to watch and a callback to fire on each message.

var capcon = require('capture-console');

// our buffer
var output = '';

// the first parameter here is the stream to capture, and the
// second argument is the function receiving the output
capcon.startCapture(process.stdout, function (stdout) {
  output += stdout;

// whatever is done here has stdout captured - but note
// that `output` is updated throughout execution


// anything logged here is no longer captured


You should be aware that all capture functions will still pass the values through to the main stdio write() functions, so logging will still go to your standard IO devices.

If this is not desirable, you can use the intercept functions. These functions are literally s/capture/intercept when compared to those shown above, and the only difference is that calls aren't forwarded through to the base implementation.

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