Map an array of arrays

Just _.map it twice:

var array = [[1, 2], [3, 4]];
var doubledArray = _.map(array, function (nested) {
    return _.map(nested, function (element) {
        return element * 2;
    });
});

Or without lodash:

var doubledArray = array.map(function (nested) {
    return nested.map(function (element) {
        return element * 2;
    });
});

Furthermore, consider using es6 arrow functions:

var doubledArray = array.map(nested => nested.map(element => element * 2));