ts-typetools

TypeTools are a collection of TypeScript type, type expressions and functions that aid writing idiomatic JavaScript in TypeScript a little easier.

Convenience Type Aliases

- Falsy represents the type of all falsy values.
- NullLike represents the null and undefined types.
- Key represents the type of all valid keys, namely number, string and symbol.
- Primitive represents the type of all primitive (not object) values, namely
string, number, boolean, symbol, null, undefined.
- Concrete represents the type of everything that is not null or undefined,
(the opposite of NullLike).

Convenience Type Expressions

- MaybePromise is a shortcut for T | Promise.
- ResolveType obtains the resolve() type of a promise T, or the
resolve() type of the return value of a function T, otherwise T itself.
- MaybeArray is a shortcut for T | Array.
- ArrayItemType obtains the element type of an array T, otherwise T
itself.
- ExtractRefine refines part of type T that extends U (like
the built-in Extract type) by interesecting that extraction with the
V type.
- ExtractOmit selectively omits properties K from a part of type
T that extends U.
- ExtractReplace selectively replaces a type in T that extends U
with V.
- UnionKeyOf obtains the union of the key types of each union component.
This is different from applying keyof to a union, which returns an
_intersection_ of the key types of each union component. While keyof returns
keys that exist in every union component, UnionKeyOf returns keys that
existing in at least one (but not necessarily all) union component.
- OmitStrict drops keys K from type T, just like native Omit, except
that here the keys are constrained to the union of keys of T (see UnionKeyOf).
- FitsSub gives Sub type back, but will require that it extends
Super.
- FitsSuper gives Super type back, but will require that Sub
extends from it.

Convenience Guards

- isConcrete() guards for the Concrete type
- isTruthy() guards for the truthy type.
- isEnum(enumObject) produces a function that guards for the enum provided by
the enumObject.
- hasKey(key) produces a function that guards for the subset of values that
are objects containing a property with the name specified in key.
- hasTag(tagOrTags, prop) produces a function that guards for the objects containing
a property named as specified in prop, whose value is one of the tags
specified. This is used to narrow discriminated unions.

Type-aware utilities

- not(fn) produces a function that returns the boolean-negated version of the
original functin's result. If the given function is a guard, it produces
an "anti-guard".
- sieve(fnMap, tagProp) produces a function that calls a different function on
the map based on the tag of input value, which is a discrimated union.
- asType(value) is an identity function that casts value into a broader
type specified in T. This is the same as if we were to save value into
a temporary const of type T, and is the opposite of doing value as T,
where value could be cast into a narrower type without assertion
(as is therefore unsafe).
- fitsType(value) is an identity function that checks that value is a
subtype of T without casting the type of value itself into T.
- readonly(value) is an identity function that

Test utilities

- expectType(value) or expectType() can be used to perform type testing,
using the quasi-human language and one of the four assertion symbols:
ExactType, SubType, SuperType and Unrelated. Examples:

``ts
// Use types
expectType().assert().toBe(ExactType);
expectType().assert<2>().toBe(SubType);
expectType().assertBase<2>().toBe(SuperType);
expectType().assert().toBe(Unrelated);

// Or use values
expectType(10).assert(10).toBe(ExactType);
expectType(10)
.assert(2 as const)
.toBe(SubType);
expectType(10)
.assertBase(2 as const)
.toBe(SuperType);
expectType(10).assert(false).toBe(Unrelated);

// Or mix them up
expectType().assert(10).toBe(ExactType);
expectType(10).assert<2>().toBe(SubType);
expectType()
.assertBase(2 as const)
.toBe(SuperType);
expectType(10).assert().toBe(Unrelated);
``