Imperative vs Declarative

declarative.js

vm.availableFields = rsDataModel.fields.map(function (field) {
    return field.childFields.length ? field.childFields : field;
}).reduce(function (prev, curr) {
    return prev.concat(curr);
}, []);

imperative.js

vm.availableFields = [];

rsDataModel.fields.forEach(function (field) {
    if (field.childFields.length) {
        field.childFields.forEach(function (childField) {
            vm.availableFields.push(childField);
        });
    }
    else {
        vm.availableFields.push(field);
    }
});