Node.js Intercept stdout

intercept-stdout captures or modifies stdout and/or stderr.

Based on this gist.

Capture

// Start capturing stdout.
var intercept = require("intercept-stdout");
var captured_text = "";

var unhook_intercept = intercept(function(text) {
  captured_text += text;
});

console.log("This text is being captured.");

// Stop capturing stdout.
unhook_intercept();

console.log("This text is not being captured.");

// This is the text that was captured.
console.log("CAPTURED:", captured_text);

Modify

// Start capturing stdout.
var intercept = require("intercept-stdout");
var modified_text = "";

var unhook_intercept = intercept(function(text) {
  modified_text += text.replace(/captured/i, "modified");
});

console.log("This text is being captured.");

// Stop capturing stdout.
unhook_intercept();

// This is the modified text.
console.log("MODIFIED:", modified_text);

Suppress output

By default, the captured text is still sent to stdout. To avoid this, return an empty string in the interceptor:

var intercept = require("intercept-stdout");

var logs = [];

var unhook_intercept = intercept(function(text) {
  logs.push(text);
  return '';
});

console.log("This text won't be sent to stdout.");

// Stop capturing stdout.
unhook_intercept();

// Strings sent to stdout
console.log("Logs:", logs);

Test

npm install
npm test

Separating Error Text

Starting in Version 0.1.2, you may now specify two interceptor callbacks. If a second interceptor callback is specified, the second callback will be invoked for stderr output.

Errors and Warnings

Versions > 0.1.1 hook both stdout and stderr. This change enables capturing of console.log, console.info, console.warn, and console.error. This change may break pre-existing interceptors if your interceptor expected output to be a full line of text.

About Colorization

Popular modules such as mocha and winston may colorize output by inserting ANSI escape codes into the output stream. Both mocha and winston make multiple calls to the output streams while colorizing a line -- in order to be robust, your code should anticipate and deal with this.