How to assert process.stderr output?

Question

How to assert process.stderr output?

I'm building a little command line tool and I hit a problem with testing.

How can test that the current process wrote "omg" in stderr?

process.stderr.write("omg")

javascript
node.js
testing
io
tdd

Answer 1

Install Mocha if you haven't:

npm install -g mocha

shouldpass.js:

process.stderr.write('omg')

shouldfail.js:

process.stdout.write('not omg on stderr')

test.js:

var exec = require('child_process').exec
  , assert = require('assert')

describe('run tests', function(){
  it('should pass', function(done) {
    exec('node ./shouldpass.js', function(err, stdout, stderr) {
      assert.equal(stderr, 'omg')
      done()    
    })
  })

  it('should fail', function(done) {
    exec('node ./shouldfail.js', function(err, stdout, stderr) {
      assert.equal(stderr, 'omg')
      done()    
    })
  })    
})

To run:

mocha test.js