Where

where(...) is the workhorse filter. It supports three argument shapes and ~50 operators.

Signatures

// 1. Equality on a keyed value:
c.where(key: string, value: any): ImmutableCollection<T>

// 2. Operator on a keyed value:
c.where(key: string, operator: ComparisonOperator, value: any): ImmutableCollection<T>

// 3. Operator on primitive items (key omitted):
c.where(operator: ComparisonOperator, value: any): ImmutableCollection<T>
// e.g. collect([1, 2, 3, 4]).where(">", 2) → [3, 4]

The wrapper looks at args.length and at whether the first arg is a known operator (Operators is a constant as const tuple of every string the switch handles).

Operators

Group	Aliases
Equals	=, equals
Not equals	!=, not, not equals
Greater than	>, gt
Greater than or equal	>=, gte
Less than	<, lt
Less than or equal	<=, lte
Substring (case-insensitive)	like, %
Not substring	not like, !%
Regex test	regex (and: passing a RegExp as the value with no operator)
Set membership	in
Not in set	not in, !in
Range (inclusive)	between, <>
Not range	not between, !between, !<>
Substring (case-sensitive on .includes)	contains
Not contains	not contains, !contains
Starts with	starts with
Not starts with	not starts with, !starts with
Ends with	ends with
Not ends with	not ends with, !ends with
Is null	null, is null
Not null	is not null, !null
Is undefined	undefined, is undefined
Not undefined	is not undefined, !undefined
Exists	exists
Not exists	not exists, !exists
Is true	true, is true
Not true	!true, is not true
Is false	false, is false
Not false	!false, is not false
typeof	is, typeof
Not typeof	is not, !is, not typeof
instanceof	instanceof, is a
Not instanceof	not instanceof, !instanceof, is not a, !is a
Empty (via @mongez/supportive-is’ isEmpty)	empty, is empty
Not empty	not empty, is not empty, !empty

Examples

// Equality
collect(users).where("active", true);

// Comparison
collect(users).where("age", ">", 25);
collect(users).where("age", "between", [20, 30]);

// String matching
collect(users).where("name", "like", "ada");          // case-insensitive substring
collect(users).where("name", "starts with", "A");
collect(users).where("name", "regex", /^A/);
collect(users).where("name", /^A/);                    // RegExp as value also works

// Set membership
collect(users).where("name", "in", ["Ada", "Bob"]);
collect(users).where("status", "!in", ["banned"]);

// Existence / null / undefined
collect(users).where("nickname", "exists");
collect(users).where("nickname", "is null");

// Type checks
collect(values).where("age", "is", "number");
collect(values).where("payload", "instanceof", ImmutableCollection);

Shorthand helpers

For common operators, there are dedicated methods that read more naturally:

Helper	Equivalent where call
whereIn(key, values) / whereIn(values)	where(key, "in", values)
whereNot(key, value) / whereNot(value)	where(key, "!=", value)
whereBetween(key, [min, max])	where(key, "between", [min, max])
whereNotBetween(key, [min, max])	where(key, "not between", [min, max])
whereEmpty(key?)	where(key, "is empty") (or applied to items directly when key is omitted)
whereNotEmpty(key?) / heavy(key?)	where(key, "is not empty")
whereNull(key?)	where(key, "is null")
whereNotNull(key?)	where(key, "is not null")
whereUndefined(key?)	where(key, "is undefined")
whereNotUndefined(key?)	where(key, "is not undefined")
whereExists(key)	where(key, "exists")
whereNotExists(key)	where(key, "not exists")

firstWhere / lastWhere

c.firstWhere(key, value): T | undefined
c.firstWhere(key, operator, value): T | undefined
c.lastWhere(key, value): T | undefined
c.lastWhere(key, operator, value): T | undefined

Equivalent to c.where(...).first() / c.where(...).last().

Behaviors worth knowing

• getItemValue(item, key) prefers item.get(key) when it exists (lets you pass model classes), otherwise checks for a direct own property on the item, then falls back to dot-notation get(item, key).
• Comparing arrays / objects uses areEqual from @mongez/reinforcements, which is a deep equality check.
• Date comparison (Date instance vs Date instance) uses +itemValue === +value (numeric time).
• Empty uses @mongez/supportive-is’ isEmpty — true for null, undefined, "", [], {}, etc.
• is undefined vs not exists: own-but-explicitly-undefined keys match where(key, "is undefined"); truly-missing keys (key absent on the object) match where(key, "not exists"). The two are distinguishable thanks to an own-property check in getItemValue before falling back to the dotted get() helper.