Type-check conditions

_$(value, "name") builds a validation chain. Finish it with .$_() to require a value or .default(fallback) to supply a value only when it is undefined. An invalid defined value still throws; the fallback is not a replacement for invalid input.

var port = _$("8080", "port").toNumber().check(v => isNumber(v) && v >= 1 && v <= 65535).$_();
var label = _$(undefined, "label").isString().default("untitled");

Condition	Meaning
isString, isNumber, isBoolean	Check the corresponding value type
isArray, isMap, isObject	Check the container type
isDate, isFunction, isRegExp, isJavaObject	Check specialized values
isInstanceOf(Type)	Check JavaScript instanceof
isNotNull()	Reject an explicitly null value
oneOf(values)	Check a scalar against an allowed list
anyOf(values)	Check that every element of an input array belongs to an allowed list
less, lessEquals, greater, greaterEquals	Numeric comparisons
check(predicate, message)	Apply an explicit predicate
isSchema(schema, message, options)	Validate against JSON Schema

Conversions such as toNumber() and toString() return the validation chain, not the converted value. Finish the chain before assigning its result.

Snapshot compatibility

A group of chained checks accepts a defined value when at least one check passes in this snapshot. Use one combined predicate when every condition must hold. Global isDef and isUnDef are not builder methods. The regex helper is regexp, and there is no inRange. See the full compatibility notes, including starts / notStarts limitations.

In the November source snapshot, the implementation of equals and notEquals tests the opposite of their names, between and betweenEquals do not return the chain, and the chain’s isUUID rejects a matching UUID. Until those upstream behaviors change, use explicit predicates:

_$("ready").check(v => v === "ready").$_();
_$(3).check(v => isNumber(v) && v >= 1 && v <= 5).$_();
_$("123e4567-e89b-12d3-a456-426614174000").check(v => isUUID(v)).$_();

These are documentation workarounds, not runtime changes. See validation, parameter validation, and the source implementation.