Guards

Guards: type guards and assertions that narrow values (the enum guards are in enum).

Export	What it does
assert	Throws when a condition is falsy, and narrows its type otherwise.
assertNever	Marks a code path as unreachable: in the default of a switch over a union, TypeScript fails when a member is not handled, and an unexpected value throws at run time.
isArray	Checks whether a value is an array, whatever its items.
isArrayOf	Checks whether a value is an array whose every item passes a type guard.
isBoolean	Checks whether a value is a boolean.
isDefined	Checks whether a value is neither null nor undefined: the opposite of isNullish.
isFiniteNumber	Checks whether a value is a finite number: neither NaN nor an infinity.
isFunction	Checks whether a value is callable: a function or a class.
isNonEmptyArray	Checks whether an array has an item, and narrows it so that its first item is defined.
isNotUndefined	Checks whether a value is not undefined; null passes, unlike with isDefined.
isNull	Checks whether a value is null; undefined does not pass.
isNullish	Checks whether a value is null or undefined: the opposite of isDefined.
isNumber	Checks whether a value is a number other than NaN.
isObject	Checks whether a value is a non-null object: plain object, array, class instance, date… isRecord accepts plain objects only.
isRecord	Checks whether a value is a plain object: an object literal or Object.create(null).
isString	Checks whether a value is a string primitive.
isUndefined	Checks whether a value is undefined; null does not pass.