Sorting an array of objects by more than one key

sort-by-more-than-one.js

var arr = [
    {first: 'Joseph', last: 'Chapman'}, 
    {first: 'Andrew', last: 'Chapman'},
    {first: '234', last: '111'},
    {first: 'Joseph', last: 'Hinge'},
    {first: 'Simon', last: 'Andrews'},
    {first: 'Robert', last: 'Hinge'}
];
    
var sortBy = function (key, minor) {
    return function (o, p) {
        var a, b;
        if (o && p && typeof o === 'object' && typeof p === 'object') {
            a = o[key];
            b = p[key];
            if (a === b) {
                return typeof minor === 'function' ? minor(o, p) : 0;
            }
            if (typeof a === typeof b) {
                return a < b ? -1 : 1;
            }
            return typeof a < typeof b ? -1 : 1;
        }
    }        
};

arr.sort(sortBy('last', sortBy('first')));

// arr is [{first: '234', last: '111'}, 
//           {first: 'Simon', last: 'Andrews'}, 
//           {first: 'Andrew', last: 'Chapman'}, 
//           {first: 'Joseph', last: 'Chapman'},
//           {first: 'Simon', last: 'Andrews'},
//           {first: 'Robert', last: 'Hinge'}]