diff --git a/src/api/categories.js b/src/api/categories.js
index 4806868f93..61071a9598 100644
--- a/src/api/categories.js
+++ b/src/api/categories.js
@@ -158,7 +158,7 @@ categoriesAPI.getTopics = async (caller, data) => {
 categoriesAPI.setWatchState = async (caller, { cid, state, uid }) => {
 	let targetUid = caller.uid;
 	let cids = Array.isArray(cid) ? cid : [cid];
-	cids = cids.map(cid => (utils.isNumber(cid) ? parseInt(cid, 10) : cid));
+	cids = cids.map(cid => String(cid));
 
 	if (uid) {
 		targetUid = uid;
@@ -170,9 +170,9 @@ categoriesAPI.setWatchState = async (caller, { cid, state, uid }) => {
 	// filter to subcategories of cid
 	let cat;
 	do {
-		cat = categoryData.find(c => !cids.includes(c.cid) && cids.includes(c.parentCid));
+		cat = categoryData.find(c => !cids.includes(String(c.cid)) && cids.includes(String(c.parentCid)));
 		if (cat) {
-			cids.push(cat.cid);
+			cids.push(String(cat.cid));
 		}
 	} while (cat);
 
diff --git a/src/api/search.js b/src/api/search.js
index 83db189de3..a5bc32f83a 100644
--- a/src/api/search.js
+++ b/src/api/search.js
@@ -22,7 +22,7 @@ searchApi.categories = async (caller, data) => {
 	data.states = (data.states || ['watching', 'tracking', 'notwatching', 'ignoring']).map(
 		state => categories.watchStates[state]
 	);
-	data.parentCid = parseInt(data.parentCid || 0, 10);
+	data.parentCid = String(data.parentCid || 0);
 
 	let cids;
 	if (data.search) {
@@ -42,15 +42,15 @@ searchApi.categories = async (caller, data) => {
 	});
 
 	if (Array.isArray(data.selectedCids)) {
-		data.selectedCids = data.selectedCids.map(cid => parseInt(cid, 10));
+		data.selectedCids = data.selectedCids.map(cid => String(cid));
 	}
 
 	let categoriesData = categories.buildForSelectCategories(visibleCategories, ['disabledClass'], data.parentCid);
 	categoriesData = categoriesData.slice(0, 1000);
 
 	categoriesData.forEach((category) => {
-		category.selected = data.selectedCids ? data.selectedCids.includes(category.cid) : false;
-		if (matchedCids.includes(category.cid)) {
+		category.selected = data.selectedCids ? data.selectedCids.includes(String(category.cid)) : false;
+		if (matchedCids.includes(String(category.cid))) {
 			category.match = true;
 		}
 	});
@@ -72,7 +72,7 @@ async function findMatchedCids(uid, data) {
 		localOnly: data.localOnly,
 	});
 
-	let matchedCids = result.categories.map(c => c.cid);
+	let matchedCids = result.categories.map(c => String(c.cid));
 	// no need to filter if all 3 states are used
 	const filterByWatchState = !Object.values(categories.watchStates)
 		.every(state => data.states.includes(state));
@@ -82,8 +82,12 @@ async function findMatchedCids(uid, data) {
 		matchedCids = matchedCids.filter((cid, index) => data.states.includes(states[index]));
 	}
 
-	const rootCids = _.uniq(_.flatten(await Promise.all(matchedCids.map(categories.getParentCids))));
-	const allChildCids = _.uniq(_.flatten(await Promise.all(matchedCids.map(categories.getChildrenCids))));
+	const rootCids = _.uniq(_.flatten(
+		await Promise.all(matchedCids.map(categories.getParentCids))
+	));
+	const allChildCids = _.uniq(_.flatten(
+		await Promise.all(matchedCids.map(categories.getChildrenCids))
+	));
 
 	return {
 		cids: _.uniq(rootCids.concat(allChildCids).concat(matchedCids)),
@@ -145,7 +149,7 @@ searchApi.roomUsers = async (caller, { query, roomId }) => {
 	roomUsers.forEach((user, index) => {
 		if (user) {
 			user.isOwner = isOwners[index];
-			user.canKick = isRoomOwner && (parseInt(user.uid, 10) !== parseInt(caller.uid, 10));
+			user.canKick = isRoomOwner && String(user.uid) !== String(caller.uid);
 		}
 	});
 
diff --git a/src/categories/index.js b/src/categories/index.js
index 948ff86226..4d772f49f9 100644
--- a/src/categories/index.js
+++ b/src/categories/index.js
@@ -104,7 +104,6 @@ Categories.getAllCidsFromSet = async function (key) {
 	}
 
 	cids = await db.getSortedSetRange(key, 0, -1);
-	cids = cids.map(cid => utils.isNumber(cid) ? parseInt(cid, 10) : cid);
 	cache.set(key, cids);
 	return cids.slice();
 };
@@ -261,7 +260,7 @@ Categories.getChildren = async function (cids, uid) {
 async function getChildrenTree(category, uid) {
 	let childrenCids = await Categories.getChildrenCids(category.cid);
 	childrenCids = await privileges.categories.filterCids('find', childrenCids, uid);
-	childrenCids = childrenCids.filter(cid => parseInt(category.cid, 10) !== parseInt(cid, 10));
+	childrenCids = childrenCids.filter(cid => String(category.cid) !== String(cid));
 	if (!childrenCids.length) {
 		category.children = [];
 		return;
@@ -280,7 +279,7 @@ Categories.getParentCids = async function (currentCid) {
 		// eslint-disable-next-line
 		cid = await Categories.getCategoryField(cid, 'parentCid');
 		if (cid) {
-			parents.unshift(cid);
+			parents.unshift(String(cid));
 		}
 	}
 	return parents;
@@ -291,12 +290,12 @@ Categories.getChildrenCids = async function (rootCid) {
 	async function recursive(keys) {
 		let childrenCids = await db.getSortedSetRange(keys, 0, -1);
 
-		childrenCids = childrenCids.filter(cid => !allCids.includes(utils.isNumber(cid) ? parseInt(cid, 10) : cid));
+		childrenCids = childrenCids.filter(cid => !allCids.includes(cid));
 		if (!childrenCids.length) {
 			return;
 		}
+		allCids.push(...childrenCids);
 		keys = childrenCids.map(cid => `cid:${cid}:children`);
-		childrenCids.forEach(cid => allCids.push(utils.isNumber(cid) ? parseInt(cid, 10) : cid));
 		await recursive(keys);
 	}
 	const key = `cid:${rootCid}:children`;
diff --git a/src/categories/search.js b/src/categories/search.js
index b0b8bef9b9..9f8c407e76 100644
--- a/src/categories/search.js
+++ b/src/categories/search.js
@@ -94,7 +94,7 @@ module.exports = function (Categories) {
 	}
 
 	async function getChildrenCids(cids, uid) {
-		const childrenCids = await Promise.all(cids.map(cid => Categories.getChildrenCids(cid)));
+		const childrenCids = await Promise.all(cids.map(Categories.getChildrenCids));
 		return await privileges.categories.filterCids('find', _.flatten(childrenCids), uid);
 	}
 };
diff --git a/src/categories/update.js b/src/categories/update.js
index 0579019e2f..2c2053f9fa 100644
--- a/src/categories/update.js
+++ b/src/categories/update.js
@@ -67,8 +67,8 @@ module.exports = function (Categories) {
 	}
 
 	async function updateParent(cid, newParent) {
-		newParent = parseInt(newParent, 10) || 0;
-		if (parseInt(cid, 10) === newParent) {
+		newParent = String(newParent || 0);
+		if (String(cid) === newParent) {
 			throw new Error('[[error:cant-set-self-as-parent]]');
 		}
 		const childrenCids = await Categories.getChildrenCids(cid);
@@ -76,7 +76,7 @@ module.exports = function (Categories) {
 			throw new Error('[[error:cant-set-child-as-parent]]');
 		}
 		const categoryData = await Categories.getCategoryFields(cid, ['parentCid', 'order']);
-		const oldParent = categoryData.parentCid;
+		const oldParent = String(categoryData.parentCid);
 		if (oldParent === newParent) {
 			return;
 		}
diff --git a/src/controllers/admin/categories.js b/src/controllers/admin/categories.js
index 54c9b276f6..96fd2336c7 100644
--- a/src/controllers/admin/categories.js
+++ b/src/controllers/admin/categories.js
@@ -52,7 +52,7 @@ categoriesController.getAll = async function (req, res) {
 	const rootCid = parseInt(req.query.cid, 10) || 0;
 	const rootChildren = await categories.getAllCidsFromSet(`cid:${rootCid}:children`);
 	async function getRootAndChildren() {
-		const childCids = _.flatten(await Promise.all(rootChildren.map(cid => categories.getChildrenCids(cid))));
+		const childCids = _.flatten(await Promise.all(rootChildren.map(categories.getChildrenCids)));
 		return [rootCid].concat(rootChildren.concat(childCids));
 	}
 
diff --git a/src/controllers/search.js b/src/controllers/search.js
index 440deb0fc5..3a2c5af54e 100644
--- a/src/controllers/search.js
+++ b/src/controllers/search.js
@@ -201,7 +201,7 @@ async function buildSelectedCategoryLabel(selectedCids) {
 			label = `[[search:categories-x, ${selectedCids.length}]]`;
 		} else if (selectedCids.length === 1 && selectedCids[0] === 'watched') {
 			label = `[[search:categories-watched-categories]]`;
-		} else if (selectedCids.length === 1 && parseInt(selectedCids[0], 10)) {
+		} else if (selectedCids.length === 1 && selectedCids[0]) {
 			const categoryData = await categories.getCategoryData(selectedCids[0]);
 			if (categoryData && categoryData.name) {
 				label = `[[search:categories-x, ${categoryData.name}]]`;
diff --git a/src/posts/category.js b/src/posts/category.js
index b87342a89f..7ce02bf490 100644
--- a/src/posts/category.js
+++ b/src/posts/category.js
@@ -40,7 +40,7 @@ module.exports = function (Posts) {
 	};
 
 	async function filterPidsBySingleCid(pids, cid) {
-		const isMembers = await db.isSortedSetMembers(`cid:${parseInt(cid, 10)}:pids`, pids);
+		const isMembers = await db.isSortedSetMembers(`cid:${cid}:pids`, pids);
 		return pids.filter((pid, index) => pid && isMembers[index]);
 	}
 };
diff --git a/src/search.js b/src/search.js
index e1a7fb4287..9993a8121d 100644
--- a/src/search.js
+++ b/src/search.js
@@ -412,8 +412,12 @@ async function getChildrenCids(data) {
 	if (!data.searchChildren) {
 		return [];
 	}
-	const childrenCids = await Promise.all(data.categories.map(cid => categories.getChildrenCids(cid)));
-	return await privileges.categories.filterCids('find', _.uniq(_.flatten(childrenCids)), data.uid);
+	const childrenCids = await Promise.all(data.categories.map(categories.getChildrenCids));
+	return await privileges.categories.filterCids(
+		'find',
+		_.uniq(_.flatten(childrenCids)),
+		data.uid
+	);
 }
 
 async function getSearchUids(data) {
diff --git a/src/socket.io/admin/categories.js b/src/socket.io/admin/categories.js
index 53c541598d..1406edb596 100644
--- a/src/socket.io/admin/categories.js
+++ b/src/socket.io/admin/categories.js
@@ -36,7 +36,7 @@ Categories.copyPrivilegesFrom = async function (socket, data) {
 
 Categories.copyPrivilegesToAllCategories = async function (socket, data) {
 	let cids = await categories.getAllCidsFromSet('categories:cid');
-	cids = cids.filter(cid => parseInt(cid, 10) !== parseInt(data.cid, 10));
+	cids = cids.filter(cid => String(cid) !== String(data.cid));
 	for (const toCid of cids) {
 		// eslint-disable-next-line no-await-in-loop
 		await categories.copyPrivilegesFrom(data.cid, toCid, data.group, data.filter);
diff --git a/src/socket.io/categories.js b/src/socket.io/categories.js
index ae2378d73a..68e9f1942b 100644
--- a/src/socket.io/categories.js
+++ b/src/socket.io/categories.js
@@ -111,7 +111,9 @@ SocketCategories.ignore = async function (socket, data) {
 
 async function ignoreOrWatch(fn, socket, data) {
 	let targetUid = socket.uid;
-	const cids = Array.isArray(data.cid) ? data.cid.map(cid => parseInt(cid, 10)) : [parseInt(data.cid, 10)];
+	const cids = Array.isArray(data.cid) ?
+		data.cid.map(cid => String(cid)) :
+		[String(data.cid)];
 	if (data.hasOwnProperty('uid')) {
 		targetUid = data.uid;
 	}
@@ -122,9 +124,9 @@ async function ignoreOrWatch(fn, socket, data) {
 	// filter to subcategories of cid
 	let cat;
 	do {
-		cat = categoryData.find(c => !cids.includes(c.cid) && cids.includes(c.parentCid));
+		cat = categoryData.find(c => !cids.includes(String(c.cid)) && cids.includes(String(c.parentCid)));
 		if (cat) {
-			cids.push(cat.cid);
+			cids.push(String(cat.cid));
 		}
 	} while (cat);