amb-test.js

var AmbTest = TestCase("AmbTest");

AmbTest.prototype.testBasic = function () {
  function beats(ix, iy, jx, jy) {
    return ix == jx || Math.abs(ix-jx) == jy-iy;
  }
  var result = ambRun(function (amb, fail) {
    var queenPos = [];
    for (var i = 0; i < 8; i++) {
      var lastPos = amb([0,1,2,3,4,5,6,7]);
      for (var j = 0; j < i; j++) {
        if (beats(queenPos[j], j, lastPos, i))
          fail();
      }
      queenPos.push(lastPos);
    }
    return queenPos;
  });

  console.log(result);
}

amb.js

function ambRun(body, failureValue) {
  var currentIndex = 0;
  var ambs = [];

  function fail() {
    throw fail;
  }

  function amb(choices) {
    // return old choices initially
    if (currentIndex < ambs.length) {
      // assuming choices are equal ambs[currentIndex][1]
      return choices[ambs[currentIndex++][0]];
    }

    if (currentIndex > ambs.length) {
      throw new Error("BUG");
    }

    if (!choices.length) {
      throw fail;
    }

    // and then initiate new choice level
    ambs[currentIndex++] = [0, choices];
    return choices[0];
  }

  do {
    try {
      // if we succeeded return
      return body(amb, fail);
    } catch (e) {
      if (e !== fail) {
        throw e;
      }
    }
    if (currentIndex != ambs.length) {
      throw new Error("BUG or non-deterministic body");
    }
    currentIndex = 0;
    for (var i = ambs.length-1; i >= 0; i--) {
      var currentAmb = ambs[i];
      // try 'incrementing' choice
      if (++currentAmb[0] < currentAmb[1].length) {
        // and stop when that succeeds
        break;
      }
      // if we run out of choice at some level, try level before that
      // while deleting this level from ambs (ambs.length assignment
      // after loop)
    }
    // if we run out of choices, abort
  } while ((ambs.length = i+1) > 0);

  return failureValue;
}