javascript + mocha: probably a closure issue with functions declared inside a for loop

To lea javascript, I implement and test classic algorithms with it.

Here is my attempt at implementing binary search and tests for it:

var assert = require('assert')

function binsearch (xs, v) {
  if (xs === undefined || xs.length === 0) { retu null }

  var lo = 0
  var hi = xs.length - 1

  while (lo <= hi) {
    var i = (lo + hi) / 2 | 0

    if (xs[i] === v) {
      retu i
    } else if (v < xs[i]) {
      hi = i - 1
    } else if (v > xs[i]) {
      lo = i + 1
    }
  }

  retu null
}

var check_bsearch = function (bsearch, xs, x, i) {
  it(bsearch.name + ' [' + xs + '] ' + x + ' ' + i, function () {
    assert.equal(bsearch(xs, x), i)
  })
}

describe('Test binsearch', function () {
  describe('on a sorted array', function() {
    var xs = []

    for (var i = 0; i < 2; ++i) {
      xs[i] = 2 * i

      for (var j = 0; j <= i; ++j) {
        check_bsearch(binsearch, xs, 2 * j, j)
        check_bsearch(binsearch, xs, 2 * j + 1, null)

        check_bsearch(binsearch, xs, -1, null)
        check_bsearch(binsearch, xs, -2, null)

        check_bsearch(binsearch, xs, 2 * i + 1, null)
        check_bsearch(binsearch, xs, 2 * i + 2, null)
      }
    }
  })
})

In order to reproduce what I am about to say, copy the code above into some bsearch.js file, install mocha.js with something like sudo npm install -g mocha and finally run the file: mocha bsearch.js

One test should be failing: the one with xs = [0], x = 2 and expected result of null. If you recreate this test separately though, it will pass.

I am suspecting that this is a closure issue. You might have noticed that I am already using a helper function check_bsearch to get the expected closure environment. However, I am clearly missing something. How do I fix the tests?