You have to bind the context to the correct function, i.e. sampleFunc like this:
sampleFunc.bind(this)
So your example would be:
var sampleFunc = function () {
// this is set to parent function context/this
// do something
};
async.each(sampleArray, sampleFunc.bind(this), function (err) {
// if any of the saves produced an error, err would equal that error
});