ДЗ 7

waitsFor.js

function waitsForMaker( timestep, timeout ) {
    var timestep = timestep || 100,
        timeout = timeout || 5000;

    return function( predicat, callback, errorback ) {
        var stepCount = Math.ceil(timeout / timestep),
            step = 0,
            interval;
        interval = setInterval( function() {
            step++;
            if (predicat()) {
                clearInterval(interval);    
                callback(); 
            } else if (step >= stepCount) {
                clearInterval(interval);
                errorback();
            }
        }, timestep);
    }
}

var waitsFor = waitsForMaker(50, 5000);

waitsFor(
        function(){
            return typeof watched.foo == 'function';
        },
        function() {
            alert('callback');
        },
        function() {
            alert('errorback');
        });

watched = {};
setTimeout( function() {
    watched.foo = function() {};
}, 3000);
//смотрим в консоли что происходит с переменной
setInterval( function() {
    console.log(typeof watched.foo == 'function');
},500);