lz-promise

Lazy evaluation for asynchronous operations to simplify writing efficient imperative control flow.

Inspired by lazy evaluation in languages like Haskell and Scala, except only for async operations.

Motivation

One pitfall of Promises is that they immediately execute with construction. When programming imperatively with async/await, there are often values resulting from IO calls that you only use if the code follows a particular path. It is difficult to write elegant code while minimizing these costly operations. This package solves this problem by deferring the execution of the Promise until its value is needed, and saving the value for later use, so it is never excecuted more than necessary.

Usage

const { lz } = require('lz-promise');
 
// creates a LazyPromise
const lazyValue = lz(() => new Promise(resolve => {
  console.log('executing');
  resolve();
));
 
// only executes when .then(), .catch(), or .finall() is called on LazyPromise
await lazyValue; // > executing
 

Examples

Early return

async function sendMessageToUser(message, lazyUser) {
  if (message.trim().length === 0) {
    throw new Error('message must have content');
  }
  const user = await lazyUser;
  user.send(message); 
  // ...
}
 
async function main() {
  const lazyUser = lz(() => api.getUser(id);
  await sendMessageToUser('hello', lazyUser);
  if (/* someCondition*/){
    // will only make API call if we haven't already
    const user = await lazyUser;
    await user.update();
  }
}

Conditional Control Flow

async function withLazy() {
  const lazyValue1 = lz(() => costlyApiCall1());
  const lazyValue2 = lz(() => costlyApiCall2());
 
  if (x) {
    // result of costlyApiCall1() is stored for next lazy usage
    console.log(await lazyValue1);
  }
 
  if (y) {
    // uses last result if available
    console.log(await lazyValue1);
    console.log(await lazyValue2);
  }
 
  if (z) {
    console.log(await lazyValue2);
  }
}

Array Operations

async function sendLastMessageToAllUsers(lazyLastMessage, users) {
  // if users is empty, no time is wasted fetching the last message
  await Promise.all(users.map(async user => user.send(await lazyMessage));
}

API

Classes

LazyPromise

Functions

lz(promiseCreator) ⇒ LazyPromise

Alias for LazyPromise.fromPromiseCreator that creates a new LazyPromise from a function that returns a Promise.

LazyPromise

Kind: global class

• LazyPromise
  • new LazyPromise(executor)
  • instance
    • .then(onFulfilled, onRejected) ⇒ Promise
    • .catch(onRejected) ⇒ Promise
    • .finally(onFinally) ⇒ Promise
  • static
    • .fromPromiseCreator(promiseCreator)
    • .resolve(value) ⇒ Promise
    • .reject(reason) ⇒ LazyPromise
    • .all(values) ⇒ LazyPromise
    • .race(values) ⇒ LazyPromise

new LazyPromise(executor)

Creates a new LazyPromise with the standard Promise constructor, except that executor is deferred until .then(), .catch(), or .finally() is called.

Param	Type	Description
executor	function	Callback used to initialize the promise. This callback is passed two arguments: a resolve callback used 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.

lazyPromise.then(onFulfilled, onRejected) ⇒ Promise

Calls the executor if it has not been executed. Attaches callbacks for the resolution and/or rejection of the resulting Promise.

Kind: instance method of LazyPromise
Returns: Promise -

A Promise for the completion of which ever callback is executed.

Param	Type	Description
onFulfilled	function	The callback to execute when the Promise is resolved.
onRejected	function	The callback to execute when the Promise is rejected.

lazyPromise.catch(onRejected) ⇒ Promise

Calls the executor if it has not been executed. Attaches a callback for only the rejection of the resulting Promise.

Kind: instance method of LazyPromise
Returns: Promise -

A Promise for the completion of the callback.

Param	Type	Description
onRejected	function	The callback to execute when the Promise is rejected.

lazyPromise.finally(onFinally) ⇒ Promise

Calls the executor if it has not been executed. Attaches a callback that is invoked when the resulting Promise is settled (fulfilled or rejected). The resolved value cannot be modified from the callback.

Kind: instance method of LazyPromise
Returns: Promise -

A Promise for the completion of the callback.

Param	Type	Description
onFinally	function	The callback to execute when the Promise is settled (fulfilled or rejected).

LazyPromise.fromPromiseCreator(promiseCreator)

Creates a new LazyPromise from a function that returns a Promise.

Kind: static method of LazyPromise

Param	Type	Description
promiseCreator	function	Function that the returns the Promise to be deferred until .then(), .catch(), or .finally() is called.

LazyPromise.resolve(value) ⇒ Promise

Creates a new resolved LazyPromise for the provided value. Equivalent to Promise.resolve() because value is already executed

Kind: static method of LazyPromise
Returns: Promise -

A promise whose internal state matches the provided value.

Param	Type	Description
value	any	The value which is resolved

LazyPromise.reject(reason) ⇒ LazyPromise

Creates a new rejected LazyPromise for the provided value. Equivalent to Promise.reject() because value is already executed

Kind: static method of LazyPromise
Returns: LazyPromise -

A promise whose internal state matches the provided value.

Param	Type	Description
reason	any	The value which is rejected

LazyPromise.all(values) ⇒ LazyPromise

Creates a LazyPromise that, when .then(), .catch(), or .finally() is called, is resolved with an array of results when all of the provided Promises resolve, or rejected when any Promise is rejected.

Kind: static method of LazyPromise
Returns: LazyPromise -

A new LazyPromise.

Param	Type	Description
values	Iterable	An iterator of Promises, values, or LazyPromises

LazyPromise.race(values) ⇒ LazyPromise

Creates a LazyPromise that, when .then(), .catch(), or .finally() is called, is resolved or rejected when any of the provided Promises are resolved or rejected that Promise's value.

Kind: static method of LazyPromise
Returns: LazyPromise -

A new LazyPromise.

Param	Type	Description
values	Iterable	An iterator of Promises, values, or LazyPromises.

lz(promiseCreator) ⇒ LazyPromise

Alias for LazyPromise.fromPromiseCreator that creates a new LazyPromise from a function that returns a Promise.

Kind: global function

Param	Type	Description
promiseCreator	function	Function that the returns the Promise to be deferred until .then(), .catch(), or .finally() is called.