Wait until a function returns true, then execute a callback

waitUntil.js

/*

Author: Grant Eagon

Function: Runs a function every number of milliseconds you specify and executes
a callback function when that function returns true.

Note: The reason for using a function instead of testing a variable is to
avoid using the eval() function.

Note 2: It's a good idea to nest your waitUntil function call inside another
function, since practical uses of waitUntil typically will need to test one or
more global variables.


Example usage: The follow set of functions uses waitUntil() to check to see
if 5 seconds have passed.

function getSecond() {
    var date = new Date;
    return date.getSeconds();
}

(function alertAfter5Seconds() {
    "use strict";
    var startSeconds = getSecond();
    waitUntil(function () {
        return (getSecond() >= startSeconds + 5) ? true : false;
    }, function () {
        alert('5 seconds have passed!');
    }, 1000);
}());

*/

// run boolFn every [delay] miliseconds, until it returns true, then callback()
function waitUntil(boolFn, callback, delay) {
    "use strict";
    // if delay is undefined or is not an integer
    delay = (typeof (delay) === 'undefined' || isNaN(parseInt(delay, 10))) ? 100 : delay;
    setTimeout(function () {
        (boolFn()) ? callback() : waitUntil(boolFn, callback, delay);
    }, delay);
}

I wrote it this way to avoid using eval. Haven't figured out how to get around that yet. I would prefer to just pass in a statement. The problem is that (unless you use eval), javascript won't check the variables in the statement for new values. It just takes the initial variable values passed to the function call.