type-guards

Contains utility functions for type guards, and combinators that build a TypeGuard out of others.

The combinators here work on bare type guards. Their api-contract counterparts (arrayGuard, optionalGuard, …) work on an ExpectedTypeGuard, a type guard paired with the text an issue message reports, and their names drop the Type this layer’s names carry.

Interfaces

Interface	Description
ObjectTypeGuardOptions	Options for objectTypeGuard.

Functions

Function	Description
arrayTypeGuard	Builds a guard accepting an array whose every item the item guard accepts.
assert	Asserts that a condition is true. Throws if it is not. Use in place of /* v8 ignore *\/ for defensive guards that should never trigger at runtime but would otherwise create uncovered branches.
assertGenericObject	Asserts that a value is a generic object, narrowing its type in place.
assertNever	Asserts at compile time that every case of a discriminated union has been handled. Place at the default branch of a switch over a union or enum. If a new variant is added to the union without a matching case, the call to assertNever fails to compile because value is no longer never. If reached at runtime (e.g. because the value was bypassed via JSON or a type assertion), it throws a descriptive Error.
assertNonNullable	Asserts that a value is not null or undefined, narrowing its type in place. Only callable when T includes null or undefined. Passing an already non-nullable type is a compile error.
ensureGenericObject	Ensures that a value is a generic object, returning it with narrowed type.
ensureNonNullable	Ensures that a value is not null or undefined and returns it with narrowed type. Only callable when T includes null or undefined. Passing an already non-nullable type is a compile error.
functionTypeGuard	Builds a guard accepting any function, narrowed to the signature TFunction. Only that the value is a function is checked: a signature cannot be checked at runtime, so the guard trusts it.
instanceOfTypeGuard	Builds a guard accepting an instance of a class, or of a class derived from it.
isBoolean	Checks whether a value is a boolean.
isFunction	Checks whether a value is a function.
isNull	Checks whether a value is null.
isNumber	Checks whether a value is a number.
isObject	Checks whether a value is an object: not null, and not a primitive or a function.
isString	Checks whether a value is a string.
isStringArray	Checks whether a value is an array of strings.
isUndefined	Checks whether a value is undefined.
literalTypeGuard	Builds a guard accepting exactly the listed values.
nullableTypeGuard	Widens a guard to also accept null.
objectTypeGuard	Builds a guard accepting an object whose every listed property its guard accepts. A property is read with Reflect.get, so an inherited one counts and an absent one reads as undefined, unless ObjectTypeGuardOptions.exactOptionalKeys switches the guard to own keys and exact optional properties.
optionalTypeGuard	Widens a guard to also accept undefined, which is what an optional property or parameter reads as when it is left out.
unionTypeGuard	Builds a guard accepting what any of the given guards accepts.

Types

Type	Description
GenericObject	A type that represents a generic object.
GuardedType	The type a TypeGuard narrows to. Distributes over a union of guards.
Literal	A value literalTypeGuard can compare against.
OptionalKeys	The keys of an object type that are optional.
PropertyTypeGuards	One type guard per property of an object type, as objectTypeGuard takes them. Every key of T must be listed. The guard of an optional property is typed to narrow to undefined as well, but a guard that refuses undefined still compiles there, so widen it with optionalTypeGuard: an absent property reads as undefined, and a guard refusing it refuses the object.
TypeGuard	A function deciding whether a value is a T.