Either

A fully typed, zero dependency implementation of the functional programming Either object for JavaScript and TypeScript.

Eithers are incredibly useful for functions that may throw an error. Throwing an error breaks the control flow of your program. The Either object is functional programming's solutions to handling functions that may error or succeed (and return a value).

Benefits of Eithers

Getting Started

Install the Module

npm i -g ether

Use the Module

import { Left, Right, Either } from "either"

const myFirstEither = Left('uh oh, something broke');
const myOtherEither = Right(42);

const alsoAnEither = Either.of(
    'on a roll',
    'right'
); // => Equivalent to Right('on a roll')

const stillAnEither = Either.of(
    new Error("Cannot read property foo of undefined"),
    'left'
); // => Equivalent to Left(new Error("Cannot read property foo of undefined");

Notes and Best Practices

Logging Options

When doing console.log(myEither); it's best to do console.log(myEither.toString()). This provides better output. JavaScript does not automatically invoke an object's toString method by default.

For convenience, there is a shorter named equivalent .toStr method.

For even more convenience, there is a .log method which will invoke console.log and the toString method for you.

const myEither = Right("Hello World");

// The below are all equivalent
console.log(myEither.toString());
console.log(myEither.toStr();
myEither.log();

Methods

/**

• Returns true if the instance is a Left. Returns false otherwise. */ isLeft(): boolean

/**

• Returns true if the instance is a Right. Returns true otherwise. */ isRight(): boolean

/**

• Returns false if the instance is a Left or if the instance is a
• Right but fails the predicateFn.
• Returns true if and only if the instance is a Right and passes
• the predicateFn. */ exists(predicateFn: (val: A) => boolean): boolean

/**

• Returns the underlying value regardless of whether the instance
• is a Left or a Right. */ get(): A

/**

• Returns the underlying value if it's a Right. Returns the
• provided argument otherwise. */ getOrElse(fallback: B): A | B

/**

• Returns the current instance if it's a Right. Returns the
• provided Either argument otherwise.
• @remarks Useful for chaining successive calls to return the first
• Right in the chain of orElses.
• @example
• const firstRightSidedEither = fnReturnsEither

• .orElse(fn2ReturnsEither())

• .orElse(fn3ReturnsEither())

• .orElse(fn4ReturnsEither())

*/ orElse(otherEither: Either): Either<A | B>

/**

• Transforms the underlying value if the instance is a Right by
• applying the provided function to the underlying value, returning
• the transformed value in a Right Either.
• Returns the instance otherwise.
• @remarks Prefer this to flatMap when the provided function does
• not return an Either. */ map(fn: (val: A) => B): Either<A | B>

/**

• A static version of map. Useful for lifting functions of type
• (val: A) => B to be a function of type
• (val: Either) => Either.
• A curried version of map. First accepts the transformation
• function, and returns a function that accepts the Either.
• @example
• const appendToString = (val: string) => val + "@gmail.com";
• // Eithers (possibly returned by other functions)
• const either = Right("johnsmith");
• const otherEither = Left("Error: name not entered");
• // Create a version of appendToString that works on values that
• // are Eithers.
• const appendToEitherString = Either.map(appendToString);
• const eitherEmailOrError = appendToEitherString(either);
• // eitherEmailOrError => Right("[email protected]")
• const eitherEmailOrError2 = appendToEitherString(otherEither);
• // eitherEmailOrError2 => Left("Error: name not entered");

*/ static map<B, A>( fn: (val: A) => B ): ( either: Either ) => Either<A | B>

/**

• An alias for Option.map. Perhaps a more accurate or descriptive
• name.
• Lifts a function of type (val: A) => B
• to be a function of type (val: Either) => Either.
• @example
• // Working with number
• const addFive = (val: number) => val + 5;
• const eight = addFive(3);
• // Working with Either
• const addFiveToEither = Either.lift(addFive);
• const eightOrError = addFiveToEither(Right(3)); */ static lift<B, A>( fn: (val: A) => B ): ( either: Either ) => Either<A | B>

/**

• Like Either.lift but for functions with an arbitrary number of
• arguments instead of just one.
• Lifts a function, with an arbitrary number of arguments, where
• each argument is not an Either, to be a function that works on
• Either versions of those arguments.
• @remarks This function has very weak type support and strict
• requirements to work correctly. Use with caution.
• @remarks The provided function must be completely curried.
• @remarks If any of the provided Either arguments are a Left, the
• Left will be returned.
• @remarks Each argument in the provided curried function must have
• the same type as its corresponding Either type. See the 2nd
• example below.
• @remarks All of the Either arguments for the provided function
• must be passed when liftN is invoked.
• @example
• Either.liftN(

• (a: number) => (b: number) => (c: number) => a + b + c,

• Right(18),

• Right(4),

• Right(6)

• ); // => Right(28)
• // Since the 2nd argument (b) is defined as an object with a
• // property age whose type is a number, the 2nd Either must be
• // an Either whose underlying value is an Object with a property
• // called age whose value is a number. This required relationship
• // is not enforced by the type system.
• Either.liftN(

• (a: number) => (b: { age: number }) => a + b.age,

• Right(78),

• Right({ age: 22 })

• ); // => Right(100)

*/ static liftN( // @ts-ignore fn, ...args: [Either, ...Either[]] ): Either

/**

• Applies the function wrapped in the current instance (as a Right)
• to the provided Either argument.
• If the instance is a Left, the instance is returned.
• If the instance is a Right, and the argument a Left, the argument
• is returned.
• If the instance is a Right, and the argument is a Right, the
• result of applying the function to the argument's underlying
• value is returned (wrapped in an Either as a Right).
• @remarks An Either is always returned.
• @remarks The Either's Right underlying value must be a function
• of the type (val: A) => B.
• @remarks Useful when the function to apply to another Either is
• itself wrapped in an Either.
• @example
• const getFunctionOrError = () => {

• return Math.random() > .5 ?

•     Right(val => val * 2) :

•     Left("bad luck today");

• }
• // These next two constants could be retrieved by calling
• // getFunctionOrError. Imagine it's been called twice and
• // returned the following two Eithers.
• const myRightEither = Right(val => val * 2);
• const myLeftEither = Left("bad luck today");
• // Everything works nicely if all the Eithers are Rights
• const resOne = myRightEither.ap(Right(42)); // => Right(84)
• // If the Either (containing the function or error) is a Left,
• // that instance (containing the error) is returned.
• const resTwo = myLeftEither.ap(Right(42));
• // => Left("bad luck today")
• // If the argument to ap is a Left, and the instance is a Right,
• // the argument is returned with no modification.
• const resThree = myRightEither.ap(Left(42)); // => Left(42)

*/ ap<B, C>(either: Either): Either<A | B | C>

/**

• Applies one of the provided functions to the instance's
• underlying value, returning the result of applying the function.
• If the instance is a Left, fnA is applied.
• If the instance is a Right, fnB is applied.
• @remarks Regardless of whether the instance is a Left or Right,
• the result of applying the function to the underlying value is
• returned.
• @example
• const double = val => val * 2;
• const triple = val => val * 3;
• Left(7).fold(double, triple); // => 14
• Right(7).fold(double, triple); // => 21

*/ fold(fnA: (val: A) => B, fnB: (val: A) => B): B

/**

• Transforms and returns the underlying value if the instance is a
• Right by applying the provided function to the underlying value.
• Returns a Left otherwise.
• @remarks Prefer this to map when the provided function returns
• an Either.
• @remarks If unsure of which method to use between map,
• flatMap, and then, then should always work. */ flatMap(fn: (val: A) => Either): Either<A | B>

/**

• A static version of flatMap. Useful for lifting functions of type
• (val: A) => Either to be a function of type
• (val: Either) => Either
• A curried version of flatMap. First accepts the transformation
• function, then the Either.
• @example
• const appendIfValid = (val: string): Either =>
• // Eithers - possibly returned by other parts of your code base.
• const either = Right("johnsmith");
• const otherEither = Left(new Error("invalid username"));
• // Create a version of appendIfValid that works on
• // Either
• const appendToEitherStrIfValid = Either.flatMap(appendIfValid);
• const emailAddressOrError = appendToEitherStrIfValid(opt);
• // emailAddressOrError => Right("[email protected]")
• const emailAddressOrError2 = appendToEitherStrIfValid(otherOpt);
• // emailAddressOrError2 => Left(new Error("invalid username"))
• // This next line is equivalent to the above and just avoids
• // storing the intermeidate constants.
• const emailAddressOrError3 = Either.flatMap(appendIfValid)(opt);

*/ static flatMap<B, A>( fn: (val: A) => Either ): (either: Either) => Either<A | B>

/**

• Usable in place of both map and flatMap.
• Accepts a function that returns either an Either or non Either
• value.
• Always returns an Either.
• Makes the Either class into a thenable.
• If the instance is a Left, the Either is returned as is.
• If the provided function returns an Either, the result of
• applying the function to the underlying value is returned.
• If the provided function returns a non Either, the result of
• applying the function to the underlying value is lifted into an
• Either and returned.
• @example
• const myEither = Right(10);
• const doubleOrError = (val: number): Either => {

• Math.random() > .5 ?

•     Right(val * 2) :

•     Left("Just not your day :(. Try again next time");

• }
• const alwaysDouble = (val: number): number => val * 2;
• // function calls can be chained with .then regarless if the
• // functions passed to then return an Either or non Either value.
• const eitherErrorOrValue = myEither.then(doubleOrError)

•                                .then(alwaysDouble);

*/ then(fn: (val: A) => B | Either): Either<A | B>

/**

• Flattens a wrapped Either.
• If the instance is a Left, the instance is returned.
• If the instance is a Right and the underlying value is an Either,
• the underlying value is returned.
• If the instance is a Right but the undnerlying value is not an
• Either, the instance is returned.
• @remarks In all cases, an Either is returned. */ flatten(): Either<A | B>

/**

• Returns the instance if the instance is a Left.
• Returns the instance if the instance is a Right and passes the
• provided filter function.
• Returns the otherEither if the instance is a Right and fails the
• provided filter function.
• @example
• Right(42).filterOrElse(

• val => val > 20,

• Left("not bigger than 20")

• ); // => Right(42)
• Left("uh oh, something broke").filterOrElse(

• val => val > 20,

• Left("not bigger than 20")

• ); // => Left("uh oh, something broke")
• Right(19).filterOrElse(

• val => val > 20,

• Left("not bigger than 20")

• ); // => Left("not bigger than 20")

*/ filterOrElse( filterFn: (either: A) => boolean, otherEither: Left ): Either | Left

/**

• Returns true if the instance's underlying value equals the
• provided argument. Returns false otherwise.
• @remarks Accepts an optional equality function for comparing two
• values for when the underlying value is not a primitive. By
• default this equality function is JavaScript's ===. */ contains( value: A, equalityFn: ( valOne: A, valTwo: A ) => boolean = ( valOne, valTwo ) => valOne === valTwo ): boolean

/**

• Swaps the type of the instance and returns the now
• swapped instance.
• If the instance is a Left, it becomes a Right.
• If the instance is a Right, it becomes a Left.
• @remarks The underlying value is not modified. */ swap(): Either

/**

• Returns an Array with the underlying value when the instance is a
• Right. Returns an empty Array otherwise. */ toArray(): [] | [A]

/**

• Returns a Set containing the underlying value when the instance
• is a Right. Returns an empty Set otherwise. */ toSet(): Set

/*

• Returns a string representation of the Either.
• Useful for console logging an instance.
• @example
• console.log(Left(-1)); // => "Left(-1)"
• console.log(Right(42)); // => "Right(42)"

*/ toString(): string

/**

• An alias for toString(); */ toStr(): string

/**

• Logs the Either to the console invoking both console.log and
• toString for you.
• Accepts an optional function (customToString) as an argument.
• customToString is a function you implement that returns a string.
• The string returned by customToString will be used in place of
• the string returned by toString method.
• customToString will have access to the Either instance as well
• but should not mutate the instance in any way (by calling
• map, flatMap, then, filter, etc).
• @example
• Right(3).log(); // => "Right(3)"
• Left('uh oh').log(); // => "Left('Uh oh')"
• const customLogger = (either: Either): string => {

• return ~~~~~~~~~~~ " + either.toStr() + " ~~~~~~~~~~";

• }
• Left(-1).log(customLogger) // => "~~~~~~~~~~ Left(-1) ~~~~~~~~~~"
• // Or defined inline and not even using the instance
• Right(3).log(() => "-- I AM HERE --"); // => "-- I AM HERE --"

*/ log(): void

/**

• Returns the instance after logging it to the console.
• Convenient to see the value of the Either in a sequence of method
• calls for debugging without having to split up the method calls.
• Accepts an optional function (customToString) as an argument.
• customToString is a function you implement that returns a string.
• The string returned by customToString will be used in place of
• the string returned by toString method.
• customToString will have access to the either instance as well
• but should not mutate the instance in any way (by calling
• map, flatMap, then, filter, etc).
• @example
• const customLogger = (either: Either): string => {

• return "!!!!!!! " + either.toStr() + " !!!!!!!";

• }
• Right(3)

• .map(val => val + 5)

• .logAndContinue() // => "Right(8)"

• .map(val => val + 2)

• .filter(val => val > 10)

• .logAndContinue(customLogger) // => "!!!!!!! Left() !!!!!!!"

• .getOrElse(-1);

*/ logAndContinue(): Either

/**

• Returns an instance of an Either using the value passed to it.
• When invoking this function, the caller must specify whether the
• instance is a Left or Right using the 2nd argument.
• Equivalent to using the Left() or Right() functions.
• @example
• const myRightEither = Either.of(42, 'right');
• // The above is equivalent to => Right(42)
• const myLeftEither = Either.of(42, 'left');
• // The above is equivalent to => Left(42);

*/ static of(val: T, type: 'left' | 'right'): Left | Right