Fix error on call.toString() where stack has fewer than 4 lines.

[kevincennis]

Fixes #1066

Purpose

Fixes #1066 by not calling String#replace on undefined, which could happen when an error stack had fewer than 4 lines (e.g. when the offending function was called point-free in Promise#then()).

Background

Here's a very simple reduced test case that reproduces the issue:

let sinon = require('sinon');

function test() {

  let stub1 = sinon.stub().returns( Promise.resolve({}) );
  let stub2 = sinon.stub();

  function run() {
    return stub1().then( stub2 );
  }

  run()
  .then( () => sinon.assert.calledTwice( stub2 ) )
  .catch( console.log );

}

setTimeout( test, 0 );

Because stub2 is invoked directly by the VM, it has nothing in its stack. That breaks call.toString(), which assumes each stack consists of at least 4 lines (the first 3 of which are Sinon-related, and the 4th being the beginning of the user's call stack).

Solution

Simply safe-guard against the String#replace() call by providing a default stack value of "unknown".

How to verify

1. Check out this branch (see github instructions below)
2. npm install
3. npm test

The new test sinonSpy.call > call.toString > does not throw when the call stack is empty will pass.

Or try the example under Background above.

[coveralls]

Coverage increased (+0.003%) to 95.375% when pulling f93cd3b on kevincennis:kce/fix-1066 into f701abe on sinonjs:master.

[fatso83]

Thanks for the work. Much appreciated!