mock method calling callback function sinon

sinon.stub(a, 'getText').yields(null, 'a string');

yields() will call the first function argument that gets passed to the stubbed function with the arguments provided (null, 'a string').

You can use callsArgWith

sinon.stub(a, 'getText').callsArgWith(1, null, 'a string')
let cb = function(err, response) {
   console.log(response); // 'a string'
}

a.getText('abc', cb)