isPromise

Determine if a value is a Promise or has a `then` method

Usage

The isPromise function checks if a value is “Promise-like” by determining if it has a then method.

import * as 
import _
_ from 'radashi'

import _
_.
function isPromise(value: any): value is PromiseLike<unknown>
export isPromise
Returns true if the value is a Promise or has a then method.
@see ― https://radashi.js.org/reference/typed/isPromise
@example 
isPromise(Promise.resolve(1)) // => true
isPromise({ then() {} }) // => true
isPromise(1) // => false
@version ― 12.1.0
isPromise({ 
then: () => void
then: () => {} }) // => true
import _
_.
function isPromise(value: any): value is PromiseLike<unknown>
export isPromise
Returns true if the value is a Promise or has a then method.
@see ― https://radashi.js.org/reference/typed/isPromise
@example 
isPromise(Promise.resolve(1)) // => true
isPromise({ then() {} }) // => true
isPromise(1) // => false
@version ― 12.1.0
isPromise(new 
var Promise: PromiseConstructor
new <unknown>(executor: (resolve: (value: unknown) => void, reject: (reason?: any) => void) => void) => Promise<unknown>
Creates a new Promise.
@param ― executor A callback used to initialize the promise. This callback is passed two arguments:
a resolve callback used to resolve the promise with a value or the result of another promise,
and a reject callback used to reject the promise with a provided reason or error.
Promise(() => {})) // => true
import _
_.
function isPromise(value: any): value is PromiseLike<unknown>
export isPromise
Returns true if the value is a Promise or has a then method.
@see ― https://radashi.js.org/reference/typed/isPromise
@example 
isPromise(Promise.resolve(1)) // => true
isPromise({ then() {} }) // => true
isPromise(1) // => false
@version ― 12.1.0
isPromise(
var Promise: PromiseConstructor
Represents the completion of an asynchronous operation
Promise.
PromiseConstructor.resolve<number>(value: number): Promise<number> (+2 overloads)
Creates a new resolved promise for the provided value.
@param ― value A promise.
@returns ― A promise whose internal state matches the provided promise.
resolve(1)) // => true
import _
_.
function isPromise(value: any): value is PromiseLike<unknown>
export isPromise
Returns true if the value is a Promise or has a then method.
@see ― https://radashi.js.org/reference/typed/isPromise
@example 
isPromise(Promise.resolve(1)) // => true
isPromise({ then() {} }) // => true
isPromise(1) // => false
@version ― 12.1.0
isPromise(
var Promise: PromiseConstructor
Represents the completion of an asynchronous operation
Promise.
PromiseConstructor.reject<never>(reason?: any): Promise<never>
Creates a new rejected promise for the provided reason.
@param ― reason The reason the promise was rejected.
@returns ― A new rejected Promise.
reject(new 
var Error: ErrorConstructor
new (message?: string) => Error
Error('nope'))) // => true

import _
_.
function isPromise(value: any): value is PromiseLike<unknown>
export isPromise
Returns true if the value is a Promise or has a then method.
@see ― https://radashi.js.org/reference/typed/isPromise
@example 
isPromise(Promise.resolve(1)) // => true
isPromise({ then() {} }) // => true
isPromise(1) // => false
@version ― 12.1.0
isPromise('hello') // => false
import _
_.
function isPromise(value: any): value is PromiseLike<unknown>
export isPromise
Returns true if the value is a Promise or has a then method.
@see ― https://radashi.js.org/reference/typed/isPromise
@example 
isPromise(Promise.resolve(1)) // => true
isPromise({ then() {} }) // => true
isPromise(1) // => false
@version ― 12.1.0
isPromise({}) // => false

This approach is useful for identifying objects that conform to the Promise interface without actually being instances of Promise. It’s particularly helpful in scenarios where:

You need to quickly check if a value is thenable without resolving it.
Performance is critical, and you want to avoid the overhead of Promise.resolve.
You’re working with custom Promise implementations or third-party libraries that use Promise-like objects.

While Promise.resolve is generally recommended for handling both Promise and non-Promise values uniformly, isPromise can be preferable when you need to make decisions based on whether a value is Promise-like without actually resolving or chaining it. This can be especially useful in type-checking scenarios or when implementing control flow that depends on whether a value is immediately available or needs to be awaited.