realm-js/docs/tutorials/query-language.md

The Realm JavaScript SDK supports querying based on a language inspired by NSPredicate.

The {@link Realm.Collection#filtered Collection.filtered()} method is used to query a Realm:

let contacts = realm.objects('Contact');
let friendsPage2 = contacts.filtered('type == "friend" AND name BEGINSWITH "B"');

It's possible to filter by linked or child objects with a keypath.

Example:

let johnsChildren = realm.Object('Contact').filtered('father.name == "John"');

Query strings can use numbered ($0, $1, ...) placeholders. The succeeding parameters contain the values. Named placeholders are not yet supported.

Example:

let merlots = wines.filtered('variety == $0 && vintage <= $1', 'Merlot', maxYear);

Conditional operators

You can use equality comparison on all property types: == and !=

Furthermore, the following can be used on numerical types: <, <=, >, >=

Example:

let oldContacts = realm.objects('Contact').filtered('age > 2');

Note that for boolean properties, you should test against true or false.

Example:

let women = realm.objects('Contact').filtered('isMale == false');

String operators

For string properties, prefix, suffix, and substring queries are supported by using the BEGINSWITH, ENDSWITH, CONTAINS and LIKE operators.

For any string operation you can append [c] to the operator to make it case insensitive.

Example:

let peopleWhoseNameContainsA = realm.objects('Contact').filtered('name CONTAINS[c] "a"');
let Johns = realm.objects('Contact').filtered('name ==[c] "john"');

You can do simple wildcard matching with LIKE which supports using ? to match a single character and * to match zero or multiple characters.

Example:

// Matches "John" and "Johnny"
let Johns = realm.objects('Contact').filtered('name LIKE "John*"');

Composition

Use parentheses and the &&/AND and ||/OR operators to compose queries. You can negate a predicate with !/NOT.

Queries on collections

When objects contain lists you can query into them using the collection operators ANY, ALL and NONE.

Example:

// Find contacts with one or more teenage friends
let teens = realm.objects('Contact').filtered('ANY friends.age < 14');

// Find contacts where all friends are older than 21
let adults = realm.objects('Contact').filtered('ALL friends.age > 21');

You can query on aggregates over properties in the lists using the aggregate operators .@count, .@avg, .@min, .@max and .@sum.

Example:

// Find contacts without friends
let teens = realm.objects('Contact').filtered('friends.@count == 0');

// Find contacts where the average age of their friends is above 40
let adults = realm.objects('Contact').filtered('friends.@avg.age > 40"');

Subqueries using the SUBQUERY operator allows you to filter the lists across multiple parameters while querying them.

Example:

// Find contacts with friends above 21 in SF
let teens = realm.objects('Contact').filtered('SUBQUERY(friends, $friend, $friend.age > 21 AND $friend.city = 'SF').@count > 0');

Backlink queries

Other objects can link to an object and you can query on that releationship using the @links and @links.ClassName.PropertyName syntax:

Example:

// Find contacts with no incomming links
let lonely = realm.objects('Contact').filtered('@links.@count == 0');

// Find contacts where someone from SF has them as friends
realm.objects('Contact').filtered('@links.Contact.friends.city == "SF"');