12.2.0 • Public • Published


    various typescript helper functions and utility types



    value tracking

    this library includes many helper types and functions such as add, subtract, and substring which allow the values to be known at compile time


    noUncheckedIndexedAccess support

    the noUncheckedIndexedAccess compiler flag is epic, but there's room for improvement. for example, the following issue:

    if (foo.length > 2) {
        const bar: string = foo[1] //error: string | undefined not assignable to string

    can be solved with lengthGreaterThan

    import { lengthGreaterThan } from '@detachhead/ts-helpers'
    if (lengthGreaterThan(foo, 2)) {
        const bar: string = foo[1] //no error, foo is casted to [string, string]

    there's also lengthLessThan, lengthGreaterOrEqual, etc.

    most of the array functions in this library keep track of the length, mostly thanks to this TupleOf utility type

    date formatter type

    this library contains a helper type and function for formatting dates using the date-fns format.

    const date = formatDate(new Date(), 'dd-MM-yyyy')
    assert(date === '01/01/2021') //compile error, wrong date format

    you can use any date format that date-fns accepts, and the FormatDate utility type will generate a template literal type to match your desired date format.

    Type Testing

    With the exactly function you can test if types or values are an exact match to a type

    const a: 1 | 2 = 1
    //values (also does a runtime assertion on the values)
    exactly(1 as number, a) // error as `1 | 2` is not an exact match of `number`
    exactly(1 as number, a as number) // no error
    exactly(1 as 1 | 2, a) // no error
    // mixed
    exactly<number>()(a) // error as `1 | 2` is not an exact match of `number`
    exactly<number>()(a as number) // no error
    exactly<1 | 2>()(a) // no error
    // types
    type Foo = 1 | 2
    exactly<1, Foo>() // error as `1 | 2` is not an exact match of `1`
    exactly<1 | 2, Foo>() // no error

    The Equals type allows you to check if two types are equal at the type level

    type Foo = Equals<number, 1 | 2> // false
    type Bar = Equals<any, 10> // false
    type Baz = Equals<unknown, never> // false

    variance modifier types

    when using the old method syntax, typescript does not check the variance on assignment:

    declare class A<T> {
        set(value: T): void
        get(): T
    const a = new A<number>()
    const b: A<unknown> = a
    a.get() // typescript thinks this is a number but it's actually a string

    for more information about how variance works, see this PR. the TL;DR is basically that arrow functions are checked more strictly than the old method syntax (for backwards compatibility reasons). this means you should be using arrow functions where possible.

    declare class A<T> {
        set: (value: T) => void
        get: () => T
    const a = new A<number>()
    const b: A<unknown> = a // error: Type 'A<number>' is not assignable to type 'A<unknown>'

    unfortunately however, arrow functions can't always be used. sometimes you need to use methods instead if, for example, you need to access super from a subclass:

    class B extends A<number> {
        get = () => super.get() // runtime error, arrow functions can't access super

    this is where variance modifiers come in

    SafeVariance / ToArrowFunction

    you can use SafeVariance<A> to enable strict variance checking on a class that has methods in it. or if for whatever reason you need to convert an individual function type to an arrow function type, you can use ToArrowFunction<A['get']>

    class B<T> extends A<T> {
        override get() {
            return super.get()
    const a = new B<number>()
    const b: SafeVariance<A<unknown>> = a // error

    UnsafeVariance / ToNonArrowFunction

    variance is only an issue when you're dealing with classes that have mutable state. if your type is immutable, you may want to disable variance checking without having to convert your shiny new arrow functions into cringe old methods.

    to do this, simply use UnsafeVariance<A> on your class, or ToNonArrowFunction<A['set']> to convert a function type:

    declare class A<T> {
        doSomethingElseThatTotallyDoesntChangeTheValue: (value: T) => void
        get: () => T
    const a = new A<number>()
    const b: UnsafeVariance<A<unknown>> = a // no error


    it goes without saying that these modifiers do not change the runtime behavior of a function. an arrow function is still an arrow function regardless of whether you use the ToNonArrowFunction type on it.



    this package is designed for typescript >=4.8. you will probably have issues trying to use it with older versions, tho it will probably still work if you set skipLibCheck to true in your tsconfig.json (which isn't recommended)


    as long as it supports es2021 you should be good. tested on:

    • nodejs >=15
    • browsers (chrome >=85)
    • deno (using esm.sh - eg. import { exactly } from 'http://esm.sh/@detachhead/ts-helpers/dist/utilityFunctions/misc')




    npm i @detachhead/ts-helpers

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