@lunaticmuch/front-matter-manipulator/lib/util/getFieldKeys.js

A utility for parsing and manipulating documents with Front Matter

export default function getKeys(obj) {
    var all = {};
    var seen = [];
    checkValue(obj);
    return Object.keys(all);
    function checkValue(value) {
        if (Array.isArray(value)) return checkArray(value);
        if (value instanceof Object) return checkObject(value);
    }

    function checkArray(array) {
        if (seen.indexOf(array) >= 0) return;
        seen.push(array);
        for (var i = 0, l = array.length; i < l; i++) {
            checkValue(array[i]);
        }
    }

    function checkObject(obj) {
        if (seen.indexOf(obj) >= 0) return;
        seen.push(obj);
        var keys = Object.keys(obj);
        for (var i = 0, l = keys.length; i < l; i++) {
            var key = keys[i];
            all[key] = true;
            checkValue(obj[key]);
        }
    }
}