What’s a stub? Name a use case.

Stubs are functions/programs that simulate the behaviours of components/modules. Stubs provide canned answers to function calls made during test cases. Also, you can assert on with what these stubs were called.

A use-case can be a file read, when you do not want to read an actual file:

var fs = require('fs');

var readFileStub = sinon.stub(fs, 'readFile', function(path, cb) {
  return cb(null, 'filecontent');
});

expect(readFileStub).to.be.called;
readFileStub.restore();