Utils Reference

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⚒️ Lou’s Code TypeScript curried functional utils.

Usage

📦 Node

Install @lou.codes/utils as a dependency:

1
pnpm add @lou.codes/utils
2
# or
3
npm install @lou.codes/utils
4
# or
5
yarn add @lou.codes/utils

Import it and use it:

1
import { get } from "@lou.codes/utils";
2

3
get("foo")({ foo: "bar" }); // "bar"

🦕 Deno

Import @lou.codes/utils using the npm: prefix, and use it directly:

1
import { get } from "npm:@lou.codes/utils";
2

3
get("foo")({ foo: "bar" }); // "bar"

🌎 Browser

Import @lou.codes/utils using esm.sh, and use it directly:

1
<script type="module">
2
	import { get } from "https://esm.sh/@lou.codes/utils";
3

4
	get("foo")({ foo: "bar" }); // "bar"
5
</script>

Useful links

📝 Documentation: TypeDoc generated documentation.
⏳ Changelog: List of changes between versions.
✅ Tests Coverage: Coveralls page with tests coverage.

Common

toString()

1
function toString<Input>(input: Input): Input extends Strigifiable ?
2
	`${Input<Input>}`
3
: Input extends object ? ReturnType<Input<Input>["toString"]>
4
: string;

Parses given value to string.

Type parameters

Type parameter
`Input` extends `Strigifiable` \| `object`

Parameters

Parameter	Type
`input`	`Input`

Returns

Input extends Strigifiable ? `${Input<Input>}` : Input extends object ? ReturnType<Input<Input>["toString"]> : string

stringified value.

Example

1
toString(10); // "10"
2
toString({}); // "[object Object]"
3
toString({ toString: _ => "test" }); // "test"

Conditions

when()

1
function when<Value, Predicated>(
2
	predicate: Single<Predicated> extends Single<never> ? Filter<Value>
3
	:	Predicate<Value, Predicated>,
4
): <TrueOutput>(
5
	truthyHandler: Unary<Predicated & Value, TrueOutput>,
6
) => <FalseInput, FalseOutput>(
7
	falsyHandler: Unary<FalseInput, FalseOutput>,
8
) => (value: Value) => TrueOutput | FalseOutput;

Curried conditional (like a functional ternary).

Type parameters

Type parameter	Value
`Value`	-
`Predicated`	`never`

Parameters

Parameter	Type	Description
`predicate`	`Single`<`Predicated`> extends `Single`<`never`> ? `Filter`<`Value`> : `Predicate`<`Value`, `Predicated`>	Predicate to check value against.

Returns

Function

Curried function with predicate in context.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Predicated` & `Value`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Value` extends `Predicated` ? `Value`<`Value`> : `Predicated`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`Value`

Returns

TrueOutput | FalseOutput

Example

1
const isEven = (value: number) => value % 2 === 0;
2
const whenIsEven = when(isEven);
3
const double = (value: number) => value * 2;
4
const doubleWhenEven = whenIsEven(double);
5
const triple = (value: number) => value * 3;
6
const tripleWhenOdd = whenIsEven(triple);
7

8
tripleWhenOdd(2); // 4
9
tripleWhenOdd(3); // 9

whenFalsy()

1
function whenFalsy<Value, Predicated>(
2
	predicate: Single<Predicated> extends Single<never> ? Filter<Value>
3
	:	Predicate<Value, Predicated>,
4
): <FalseInput, FalseOutput>(
5
	falsyHandler: Unary<FalseInput, FalseOutput>,
6
) => (value: Value) => (Predicated & Value) | FalseOutput;

Like when, but when is falsy the value is returned as is.

Type parameters

Type parameter	Value
`Value`	-
`Predicated`	`never`

Parameters

Parameter	Type	Description
`predicate`	`Single`<`Predicated`> extends `Single`<`never`> ? `Filter`<`Value`> : `Predicate`<`Value`, `Predicated`>	Predicate to check value against.

Returns

Function

Curried function with predicate in context.

Type parameters

Type parameter	Value
`FalseInput`	`Value` extends `Predicated` ? `Value`<`Value`> : `Predicated`
`FalseOutput`	`Predicated` & `Value`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`Value`

Returns

Predicated & Value | FalseOutput

Example

1
const isEven = (value: number) => value % 2 === 0;
2
const whenIsOdd = whenFalsy(isEven);
3
const double = (value: number) => value * 2;
4
const doubleWhenOdd = whenIsOdd(double);
5

6
doubleWhenOdd(2); // 2
7
doubleWhenOdd(3); // 6

whenIsArray()

1
function whenIsArray<TrueOutput>(
2
	truthyHandler: Unary<ReadOnlyArray<unknown>, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is an Array.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`ReadOnlyArray`<`unknown`>, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`ReadOnlyArray`<`unknown`>
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsAsyncIterable()

1
function whenIsAsyncIterable<TrueOutput>(
2
	truthyHandler: Unary<Readonly<AsyncIterable<unknown>>, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is an AsyncIterable.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Readonly`<`AsyncIterable`<`unknown`>>, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Readonly`<`AsyncIterable`<`unknown`>>
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsBigInt()

1
function whenIsBigInt<TrueOutput>(
2
	truthyHandler: Unary<bigint, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a BigInt.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`bigint`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`bigint`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsBoolean()

1
function whenIsBoolean<TrueOutput>(
2
	truthyHandler: Unary<boolean, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Boolean.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`boolean`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`boolean`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsDate()

1
function whenIsDate<TrueOutput>(
2
	truthyHandler: Unary<Date, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Date.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Date`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Date`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsFalsy()

1
function whenIsFalsy<TrueOutput>(
2
	truthyHandler: Unary<Falsy, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Falsy value.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Falsy`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Falsy`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsFunction()

1
function whenIsFunction<TrueOutput>(
2
	truthyHandler: Unary<Function, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Function.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Function`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Function`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsIsomorphicIterable()

1
function whenIsIsomorphicIterable<TrueOutput>(
2
	truthyHandler: Unary<IsomorphicIterable<unknown>, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is an AsynchronousIterable.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`IsomorphicIterable`<`unknown`>, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`IsomorphicIterable`<`unknown`>
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsIterable()

1
function whenIsIterable<TrueOutput>(
2
	truthyHandler: Unary<Readonly<Iterable<unknown>>, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is an Iterable.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Readonly`<`Iterable`<`unknown`>>, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Readonly`<`Iterable`<`unknown`>>
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsNull()

1
function whenIsNull<TrueOutput>(
2
	truthyHandler: Unary<null, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is null.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`null`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`null`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsNullish()

1
function whenIsNullish<TrueOutput>(
2
	truthyHandler: Unary<Nullish, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Nullish value.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Nullish`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Nullish`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsNumber()

1
function whenIsNumber<TrueOutput>(
2
	truthyHandler: Unary<number, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Number.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`number`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`number`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsObject()

1
function whenIsObject<TrueOutput>(
2
	truthyHandler: Unary<object, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is an Object.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`object`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`object`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsPromise()

1
function whenIsPromise<TrueOutput>(
2
	truthyHandler: Unary<Promise<unknown>, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Promise.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Promise`<`unknown`>, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Promise`<`unknown`>
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsPropertyKey()

1
function whenIsPropertyKey<TrueOutput>(
2
	truthyHandler: Unary<PropertyKey, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a PropertyKey.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`PropertyKey`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`PropertyKey`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsPrototypeOfObject()

1
function whenIsPrototypeOfObject<TrueOutput>(
2
	truthyHandler: Unary<never, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: object) => TrueOutput | FalseOutput;

Conditional handler for when something has an Object prototype.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`never`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`never`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`object`

Returns

TrueOutput | FalseOutput

whenIsRegExp()

1
function whenIsRegExp<TrueOutput>(
2
	truthyHandler: Unary<RegExp, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a RegExp.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`RegExp`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`RegExp`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsSafeInteger()

1
function whenIsSafeInteger<TrueOutput>(
2
	truthyHandler: Unary<never, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: number) => TrueOutput | FalseOutput;

Conditional handler for when a number is a SafeInteger.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`never`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`never`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`number`

Returns

TrueOutput | FalseOutput

whenIsString()

1
function whenIsString<TrueOutput>(
2
	truthyHandler: Unary<string, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a String.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`string`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`string`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsSymbol()

1
function whenIsSymbol<TrueOutput>(
2
	truthyHandler: Unary<symbol, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Symbol.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`symbol`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`symbol`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsTruthy()

1
function whenIsTruthy<TrueOutput>(
2
	truthyHandler: Unary<Readonly<unknown>, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is a Truthy value.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Readonly`<`unknown`>, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`Readonly`<`unknown`>
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenIsUndefined()

1
function whenIsUndefined<TrueOutput>(
2
	truthyHandler: Unary<undefined, TrueOutput>,
3
): <FalseInput, FalseOutput>(
4
	falsyHandler: Unary<FalseInput, FalseOutput>,
5
) => (value: unknown) => TrueOutput | FalseOutput;

Conditional handler for when something is undefined.

Type parameters

Type parameter
`TrueOutput`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`undefined`, `TrueOutput`>

Returns

Function

Type parameters

Type parameter	Value
`FalseInput`	`undefined`
`FalseOutput`	`TrueOutput`

Parameters

Parameter	Type
`falsyHandler`	`Unary`<`FalseInput`, `FalseOutput`>

Returns

Function

Parameters

Parameter	Type
`value`	`unknown`

Returns

TrueOutput | FalseOutput

whenTruthy()

1
function whenTruthy<Value, Predicated>(
2
	predicate: Single<Predicated> extends Single<never> ? Filter<Value>
3
	:	Predicate<Value, Predicated>,
4
): <Output>(
5
	truthyHandler: Unary<Predicated & Value, Output>,
6
) => (value: Value) => Value | Output;

Like when, but when is falsy the value is returned as is.

Type parameters

Type parameter	Value
`Value`	-
`Predicated`	`never`

Parameters

Parameter	Type	Description
`predicate`	`Single`<`Predicated`> extends `Single`<`never`> ? `Filter`<`Value`> : `Predicate`<`Value`, `Predicated`>	Predicate to check value against.

Returns

Function

Curried function with predicate in context.

Type parameters

Type parameter
`Output`

Parameters

Parameter	Type
`truthyHandler`	`Unary`<`Predicated` & `Value`, `Output`>

Returns

Function

Parameters

Parameter	Type
`value`	`Value`

Returns

Value | Output

Example

1
const isEven = (value: number) => value % 2 === 0;
2
const whenIsEven = whenTruthy(isEven);
3
const double = (value: number) => value * 2;
4
const doubleWhenEven = whenIsEven(double);
5

6
doubleWhenEven(2); // 4
7
doubleWhenEven(3); // 3

Functions

always()

1
function always<Input>(input: Input): () => Input;

Returns a function that always returns the same value.

Type parameters

Type parameter
`Input`

Parameters

Parameter	Type
`input`	`Input`

Returns

Function

Function that always return the given value.

Returns

Input

Example

1
const alwaysFoo = always("foo");
2
const fillWithFoo = map(alwaysFoo);
3

4
fillWithFoo([0, 1, 2]); // ["foo", "foo", "foo"]

applyTo()

1
function applyTo<Input>(
2
	input: Input,
3
): <Output>(unary: Unary<Input, Output>) => Output;

Takes a value and applies a function to it.

Type parameters

Type parameter
`Input`

Parameters

Parameter	Type
`input`	`Input`

Returns

Function

Function that expects a function that will receive the input.

Type parameters

Type parameter
`Output`

Parameters

Parameter	Type
`unary`	`Unary`<`Input`, `Output`>

Returns

Output

Example

1
const applyTo10 = applyTo(10);
2

3
applyTo10((value: number) => value * 2); // 20
4
applyTo10((value: number) => value / 2); // 5

entryKey()

1
function entryKey<InputEntry>(entry: InputEntry): EntryKey<InputEntry>;

Get key of an Entry.

Type parameters

Type parameter
`InputEntry` extends `Entry`

Parameters

Parameter	Type	Description
`entry`	`InputEntry`	Entry to get key from.

Returns

EntryKey<InputEntry>

Key of an Entry.

Example

1
entryKey(["key", "value"]); // "key"

entryValue()

1
function entryValue<InputEntry>(entry: InputEntry): EntryValue<InputEntry>;

Get value of an Entry.

Type parameters

Type parameter
`InputEntry` extends `Entry`

Parameters

Parameter	Type	Description
`entry`	`InputEntry`	Entry to get vale from.

Returns

EntryValue<InputEntry>

Value of an Entry.

Example

1
entryKey(["key", "value"]); // "value"

flip()

1
function flip<UnaryToFlip>(
2
	unary: UnaryToFlip,
3
): (
4
	...secondArgument: Parameters<UnaryOutput<UnaryToFlip>>
5
) => (
6
	...firstArgument: Parameters<UnaryToFlip>
7
) => UnaryOutput<UnaryOutput<UnaryToFlip>>;

Flips arguments of a curried function.

Type parameters

Type parameter
`UnaryToFlip` extends `Unary`<`never`, `Unary`<`never`, `unknown`>>

Parameters

Parameter	Type
`unary`	`UnaryToFlip`

Returns

Function

Curried function with unary in context.

Parameters

Parameter	Type
…`secondArgument`	`Parameters`<`UnaryOutput`<`UnaryToFlip`>>

Returns

Function

Parameters

Parameter	Type
…`firstArgument`	`Parameters`<`UnaryToFlip`>

Returns

UnaryOutput<UnaryOutput<UnaryToFlip>>

Example

1
const concat = (first: string) => (second: string) => `${first}${second}`;
2
const flippedConcat = flip(concat);
3

4
flippedConcat("1")("2"); // "21"

identity()

1
function identity<Input>(input: Input): Input;

Identity function.

Type parameters

Type parameter
`Input`

Parameters

Parameter	Type
`input`	`Input`

Returns

Input

Same value given.

Example

1
identity("foo"); // "foo"

memoize()

1
function memoize<MemoizedFunction>(unary: MemoizedFunction): MemoizedFunction;

Memoize function return values for expensive operations.

Type parameters

Type parameter
`MemoizedFunction` extends `Unary`<`never`, `unknown`>

Parameters

Parameter	Type	Description
`unary`	`MemoizedFunction`	Function to memoize.

Returns

MemoizedFunction

Curried function with unary in context.

Example

1
const expensiveOperation = (value: number) => value * 2;
2
const memoizedOperation = memoize(expensiveOperation);
3

4
memoizedOperation(2); // 4
5
memoizedOperation(2); // 4 (cached)

negate()

1
function negate<Item, Predicated>(
2
	predicate: Single<Predicated> extends Single<never> ? Filter<Item>
3
	:	Predicate<Item, Predicated>,
4
): Single<Predicated> extends Single<never> ? Unary<Item, boolean>
5
:	Predicate<Item, Exclude<Item, Predicated>>;

Takes a function and returns a copy with negated output.

Type parameters

Type parameter	Value
`Item`	-
`Predicated`	`never`

Parameters

Parameter	Type	Description
`predicate`	`Single`<`Predicated`> extends `Single`<`never`> ? `Filter`<`Item`> : `Predicate`<`Item`, `Predicated`>	Function to be negated.

Returns

Single<Predicated> extends Single<never> ? Unary<Item, boolean> : Predicate<Item, Exclude<Item, Predicated>>

Source function with negated output.

Example

1
const isPositive = (value: number) => value >= 0;
2
const isNegative = not(isPositive);
3

4
isPositive(1); // true
5
isNegative(1); // false

once()

1
function once<OnceFunction>(
2
	unary: OnceFunction,
3
): (
4
	value: UnaryInput<OnceFunction>,
5
) => UnaryOutput<OnceFunction> | UnaryOutput<OnceFunction>;

Runs a function once, and after that returns always the same value.

Type parameters

Type parameter
`OnceFunction` extends `Unary`<`never`, `unknown`>

Parameters

Parameter	Type	Description
`unary`	`OnceFunction`	Function to run once.

Returns

Function

Curried function with unary in context.

Parameters

Parameter	Type
`value`	`UnaryInput`<`OnceFunction`>

Returns

UnaryOutput<OnceFunction> | UnaryOutput<OnceFunction>

Example

1
const expensiveOperation = (value: number) => value * 2;
2
const onceOperation = once(expensiveOperation);
3

4
onceOperation(2); // 4
5
onceOperation(3); // 4 (cached)

tap()

1
function tap<Input>(
2
	tapper: Unary<Input, unknown>,
3
): <Tapped>(tapped: Tapped) => Tapped;

Tap into a value before calling a function.

Type parameters

Type parameter
`Input`

Parameters

Parameter	Type	Description
`tapper`	`Unary`<`Input`, `unknown`>	Tapper function to be called with the value.

Returns

Function

Curried function with tapper in context.

Type parameters

Type parameter
`Tapped` extends `Unary`<`Input`, `unknown`>

Parameters

Parameter	Type
`tapped`	`Tapped`

Returns

Tapped

Example

1
const log = tap(console.log);
2
const double = (value: number) => value * 2;
3
const doubleAndLog = log(double);
4

5
doubleAndLog(2); // 4 (returns and logs 4)

thunk()

1
function thunk<Input, Output>(
2
	unary: Unary<Input, Output>,
3
): (input: Input) => () => Output;

Delayed evaluation function.

Type parameters

Type parameter
`Input`
`Output`

Parameters

Parameter	Type
`unary`	`Unary`<`Input`, `Output`>

Returns

Function

Function that will run the given function when called.

Parameters

Parameter	Type
`input`	`Input`

Returns

Function

Returns

Output

Example

1
const always = thunk(id);
2
const alwaysFoo = always("foo");
3
alwaysFoo(); // "foo"

Numbers

cryptoNumber()

1
function cryptoNumber(seed: string | Numeric): Promise<number>;

Generates number using SubtleCrypto#digest and the given seed.

⚠️ IMPORTANT: This only works in secure contexts (HTTPS/Node).

Parameters

Parameter	Type	Description
`seed`	`string` \| `Numeric`	Seed to be used to generate random numbers.

Returns

Promise<number>

Pseudo-random number from seed.

Example

1
const seededRandom1 = await random("some seed");
2
const seededRandom2 = await random("some seed");
3

4
seededRandom1 === seededRandom2; // true because it has the same seed

See

SubtleCrypto#digest

Objects

awaitableHandler()

1
function awaitableHandler<Value, Output>(
2
	handler: (awaitable: Value) => Output,
3
): <AwaitableValue>(
4
	awaitable: AwaitableValue,
5
) => AwaitableValue extends Promise<Value> ? Promise<Output> : Output;

If the given value is a promise, the handler is called when is resolved, otherwise the handler is called directly with the value.

Type parameters

Type parameter
`Value`
`Output`

Parameters

Parameter	Type	Description
`handler`	(`awaitable`: `Value`) => `Output`	Handler function to be called with the value.

Returns

Function

Curried function with handler in context.

Type parameters

Type parameter
`AwaitableValue`

Parameters

Parameter	Type
`awaitable`	`AwaitableValue`

Returns

AwaitableValue extends Promise<Value> ? Promise<Output> : Output

Example

1
const double = maybePromiseHandler((value: number) => value * 2);
2

3
double(2); // 4
4
double(Promise.resolve(2)); // Promise<4>

get()

1
function get<Key>(
2
	key: Key,
3
): <Source>(object: Source) => Source[Key & keyof Source];

Get the value of a property in an object.

Type parameters

Type parameter
`Key` extends `PropertyKey`

Parameters

Parameter	Type
`key`	`Key`

Returns

Function

Curried function with key in context.

Type parameters

Type parameter
`Source` extends `Readonly`<`Record`<`Key`, `unknown`>>

Parameters

Parameter	Type
`object`	`Source`

Returns

Source[Key & keyof Source]

Example

1
const getFoo = get("foo");
2

3
getFoo({ foo: "bar" }); // "bar"
4
getFoo({}); // undefined

mutate()

1
function mutate<Update>(
2
	update: Update,
3
): <Target>(target: Target) => Target & Update;

Function to encapsulate object mutations.

Type parameters

Type parameter
`Update` extends `object`

Parameters

Parameter	Type	Description
`update`	`Update`	Update to apply to given target.

Returns

Function

Curried function with update in context.

Type parameters

Type parameter
`Target` extends `object`

Parameters

Parameter	Type
`target`	`Target`

Returns

Target & Update

Example

1
const state = { a: 1 };
2
mutate(set("a")(2))(state);
3
console.log(state); // { a: 2 }

omit()

1
function omit<Properties>(
2
	...properties: Properties
3
): <Source>(object: Source) => Omit<Source, Properties[number]>;

Omits given properties from an object.

Type parameters

Type parameter
`Properties` extends `ReadOnlyArray`<`PropertyKey`>

Parameters

Parameter	Type	Description
…`properties`	`Properties`	Properties to omit.

Returns

Function

Curried function with properties in context.

Type parameters

Type parameter
`Source` extends `Readonly`<`Record`<`PropertyKey`, `unknown`>>

Parameters

Parameter	Type
`object`	`Source`

Returns

Omit<Source, Properties[number]>

Example

1
const object = { a: 1, b: 2, c: 3 };
2
const omitAC = omit("a", "c");
3
const result = omitAC(object); // { b: 2 }

pick()

1
function pick<Properties>(
2
	...properties: Properties
3
): <Source>(object: Source) => Pick<Source, Properties[number]>;

Picks given properties from an object.

Type parameters

Type parameter
`Properties` extends `ReadOnlyArray`<`PropertyKey`>

Parameters

Parameter	Type	Description
…`properties`	`Properties`	Properties to pick.

Returns

Function

Curried function with properties in context.

Type parameters

Type parameter
`Source` extends `Readonly`<`Record`<`PropertyKey`, `unknown`>>

Parameters

Parameter	Type
`object`	`Source`

Returns

Pick<Source, Properties[number]>

Example

1
const object = { a: 1, b: 2, c: 3 };
2
const pickAC = pick("a", "c");
3
const result = pickAC(object); // { a: 1, c: 3 }

set()

1
function set<Key>(
2
	key: Key,
3
): <Value>(
4
	value: Value,
5
) => <Source>(
6
	object: Source,
7
) => Omit<Source, Key> & Readonly<Record<Key, Value>>;

Set the value of a property in an object (read only).

Type parameters

Type parameter
`Key` extends `PropertyKey`

Parameters

Parameter	Type
`key`	`Key`

Returns

Function

Curried function with key in context.

Type parameters

Type parameter
`Value`

Parameters

Parameter	Type
`value`	`Value`

Returns

Function

Type parameters

Type parameter
`Source` extends `object`

Parameters

Parameter	Type
`object`	`Source`

Returns

Omit<Source, Key> & Readonly<Record<Key, Value>>

Example

1
const setFoo = set("foo");
2

3
setFoo("baz")({ foo: "bar" }); // { foo: "baz" }
4
setFoo("baz")({ bar: "foo" }); // { bar: "foo", foo: "baz" }
5
setFoo("baz")({}); // { foo: "baz" }

Promises

otherwise()

1
function otherwise<Output>(
2
	handler: Unary<unknown, Output | PromiseLike<Output>>,
3
): <PromisedValue>(
4
	promise: Promise<PromisedValue>,
5
) => Promise<Output | PromisedValue>;

Sets a promise catch handler.

Type parameters

Type parameter
`Output`

Parameters

Parameter	Type
`handler`	`Unary`<`unknown`, `Output` \| `PromiseLike`<`Output`>>

Returns

Function

Curried function with handler set in context.

Type parameters

Type parameter
`PromisedValue`

Parameters

Parameter	Type
`promise`	`Promise`<`PromisedValue`>

Returns

Promise<Output | PromisedValue>

Example

1
const otherwiseLog = otherwise(console.error);
2

3
otherwiseLog(fetch("/api")); // Logs on error.

then()

1
function then<HandlerInput, Output>(
2
	handler: Unary<HandlerInput, Output | PromiseLike<Output>>,
3
): <PromiseInstance>(promise: PromiseInstance) => Promise<Output>;

Sets a promise then handler.

Type parameters

Type parameter
`HandlerInput`
`Output`

Parameters

Parameter	Type
`handler`	`Unary`<`HandlerInput`, `Output` \| `PromiseLike`<`Output`>>

Returns

Function

Curried function with handler set in context.

Type parameters

Type parameter
`PromiseInstance` extends `Promise`<`HandlerInput`>

Parameters

Parameter	Type
`promise`	`PromiseInstance`

Returns

Promise<Output>

Example

1
const thenJson = then((response: Response) => response.json());
2

3
thenJson(fetch("/api")); // Parsed response