$toLower (Lowercase)
Last updated: 2026-10-09
$toLower (Lowercase)
$toLower converts a string to lowercase — a fundamental normalization operator. It is used to unify storage formats, build case-insensitive query keys (with indexes), or clean user input. It accepts strings only; other types throw, so convert with $toString first. It is the inverse of $toUpper. Characters without case (like Chinese) pass through unchanged.
| Category | String |
|---|---|
| MongoDB Version | 1.0+ |
| Example Language | mongosh (Shell) |
📝 Syntax
{ $toLower: <string-expression> }⚙️ Parameters
| string | Required; the string expression to lowercase. |
|---|
Returns：The lowercased string.
▶ Example
Run the commands below in your local mongosh (requires a running mongod):
// $toLower: normalize emails to lowercase
db.users.aggregate([
{ $project: { emailLower: { $toLower: '$email' } } }
])Output：
{ "_id": 1, "emailLower": "alice@example.com" }
{ "_id": 2, "emailLower": "bob@example.com" }📌 Notes
- Strings only — convert numbers and dates first
- Inverse of $toUpper
- Characters without case, like Chinese, pass through unchanged
🏷️ Related Tags
💡 Related Cases
More hands-on cases coming soon.
📚 Related Tutorials
❓ FAQ
QDoes it affect Chinese?
AChinese has no case — characters are returned unchanged.
QDoes lowercasing speed up queries?
AStoring lowercase uniformly lets indexes serve exact matches more efficiently.
QWhat happens when used with $toUpper?
AConverting back and forth restores the original — no extra effect.
