isResult

Check if a value is a Result tuple

Usage

Check if a value is a Result tuple.

Don’t know what that is? Read the Result section further down.

import * as _ from 'radashi'

_.isResult([undefined, 42]) // => true
_.isResult([new Error(), undefined]) // => true

// Tuple must be of length 2.
_.isResult([new Error()]) // => false
_.isResult([undefined, true, undefined]) // => false

// Non-tuple values are false.
_.isResult([]) // => false
_.isResult({}) // => false
_.isResult(null) // => false

// Result tuples cannot have both a value and an error.
_.isResult([new Error(), true]) // => false

Also see the related isResultOk and isResultErr functions.

Types In-Depth

Result

“Results” are tuples of 2 elements (an error and a result value).

The first element is always the error, or undefined if the operation was successful.
The second element is always the result value, unless an error occurred.
These tuples are represented by the Result<TResult, TError> type.
A default error type of Error is used when no error type is explicitly defined (e.g. Result<string>).

Ok and Err

There are 2 types of result: Ok<TResult> and Err<TError>.

The Ok type represents a successful operation. It’s a tuple of [undefined, TResult].
The Err type represents a failed operation. It’s a tuple of [TError, undefined].

The names “Ok” and “Err” are inspired by Rust’s std::result module.

To check for an Ok result, do this:

declare const value: unknown

if (isResult(value) && value[0] == null) {
  value // <-- now an Ok<unknown> type
  value[1] // <-- This is the resulting value!
}

To check for an Err result, do this:

declare const value: unknown

if (isResult(value) && value[0] != null) {
  value // <-- now an Err<Error> type
  value[0] // <-- This is the error!
}

You can also use the isResultOk and isResultErr functions to check for Ok and Err results respectively.