Anonymous functions returning anonymous functions that return what you need.

example.js

var qty = parseInt(element.find('.qty').val()) || 0;

var possiblePrices = tierPrices[productId];
var tierPrice = jQuery.grep(possiblePrices , (function(qty) {
    var qty = qty || 1;
    return function(possiblePrice) {
        return (possiblePrice.qty_min <= qty)
            && ((possiblePrice.qty_max > qty) || (possiblePrice.qty_max === 'infinity'));
    }
})(qty))[0];