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

The Realm JavaScript SDK supports querying based on a language inspired by [NSPredicate](https://realm.io/news/nspredicate-cheatsheet/). 

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

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

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

Example:
```JS
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:
```JS
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:
```JS
// 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:
```JS
// 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:
```JS
// Find contacts without friends
let lonely = 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:
```JS
// 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.
If the relationship of the LinkingObject is named, use the name in the query just like you would use any other property for readability.
If the relationship is not named, you can use the `@links.ClassName.PropertyName` syntax where `ClassName.PropertyName` describes the forward relationship.

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

// Find contacts with no incomming links (across all linked properties)
let isolated = realm.objects('Contact').filtered('@links.@count == 0');

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