eslint-plugin-promise

What is eslint-plugin-promise?

The eslint-plugin-promise package is an ESLint plugin that enforces best practices and common idioms for dealing with promises in JavaScript. It provides a set of rules that catch common mistakes and enforce conventions when working with promises.

What are eslint-plugin-promise's main functionalities?

Avoiding creation of new promises outside of utility libraries

This rule ensures that the executor function of a new Promise has the parameters named 'resolve' and 'reject'.

/* eslint promise/param-names: 'error' */
new Promise((resolve, reject) => { /* executor function */ });

Ensuring that each time a then() is applied to a promise, a catch() is applied as well

This rule enforces the use of catch() on unhandled promises.

/* eslint promise/catch-or-return: 'error' */
promise.then(function(data) {
  // handle data
}).catch(function(error) {
  // handle error
});

Enforcing the return of a promise in certain contexts

This rule ensures that promise chains always return a value, preventing silent failures and unhandled rejections.

/* eslint promise/always-return: 'error' */
function foo() {
  return doSomething().then(function() {
    // do something else
    return 'result';
  });
}

Other packages similar to eslint-plugin-promise

eslint-plugin-async-await

This plugin provides ESLint rules for async/await syntax, which is related to promises. It helps to enforce best practices and catch common mistakes when using async/await, but it does not focus on promises directly.

eslint-plugin-standard

Part of the 'eslint-plugin-standard' plugin includes rules that are applicable to promises, as it adheres to the JavaScript Standard Style. However, it covers a broader range of JavaScript features and is not solely focused on promises.

README

eslint-plugin-promise

Enforce best practices for JavaScript promises.

Installation

You'll first need to install ESLint:

$ npm i eslint --save-dev

Next, install eslint-plugin-promise:

$ npm install eslint-plugin-promise --save-dev

Note: If you installed ESLint globally (using the -g flag) then you must also install eslint-plugin-promise globally.

Usage

Add promise to the plugins section of your .eslintrc configuration file. You can omit the eslint-plugin- prefix:

{
    "plugins": [
        "promise"
    ]
}

Then configure the rules you want to use under the rules section.

{
    "rules": {
        "promise/always-return": 2,
        "promise/no-return-wrap": 2,
        "promise/param-names": 2,
        "promise/catch-or-return": 2,
        "promise/no-native": 0,
    }
}

Rules

Promise Rules

• catch-or-return Enforces the use of catch on un-returned promises.
• no-return-wrap Avoid wrapping values in Promise.resolve or Promise.reject when not needed.
• param-names Enforce consistent param names when creating new promises.
• always-return Return inside each then to create readable and reusable Promise chains.
• no-native In an ES5 environment, make sure to create a Promise constructor before using.
• no-nesting Avoid nested .then() or .catch() statements
• no-promise-in-callback Avoid using promises inside of callbacks
• no-callback-in-promise Avoid calling cb() inside of a then()

Async/Await Rules

• prefer-await-to-then Prefer await to then() for reading Promise values
• prefer-await-to-callbacks Prefer async/await to the callback pattern

Rule: catch-or-return

Ensure that each time a then() is applied to a promise, a catch() is applied as well. Exceptions are made if you are returning that promise.

Valid

myPromise.then(doSomething).catch(errors);
myPromise.then(doSomething).then(doSomethingElse).catch(errors);
function doSomethingElse() { return myPromise.then(doSomething) }

Invalid

myPromise.then(doSomething);
myPromise.then(doSomething, catchErrors); // catch() may be a little better
function doSomethingElse() { myPromise.then(doSomething) }

Options

allowThen

You can pass an { allowThen: true } as an option to this rule to allow for .then(null, fn) to be used instead of catch() at the end of the promise chain.

terminationMethod

You can pass a { terminationMethod: 'done' } as an option to this rule to require done() instead of catch() at the end of the promise chain. This is useful for many non-standard Promise implementations.

You can also pass an array of methods such as { terminationMethod: ['catch', 'asCallback', 'finally'] }.

This will allow any of

Promise.resolve(1).then(() => { throw new Error('oops') }).catch(logerror)
Promise.resolve(1).then(() => { throw new Error('oops') }).asCallback(cb)
Promise.resolve(1).then(() => { throw new Error('oops') }).finally(cleanUp)

Rule: always-return

Ensure that inside a then() you make sure to return a new promise or value. See http://pouchdb.com/2015/05/18/we-have-a-problem-with-promises.html (rule #5) for more info on why that's a good idea.

We also allow someone to throw inside a then() which is essentially the same as return Promise.reject().

Valid

myPromise.then((val) => val * 2));
myPromise.then(function(val) { return val * 2; });
myPromise.then(doSomething); // could be either
myPromise.then((b) => { if (b) { return "yes" } else { return "no" } });

Invalid

myPromise.then(function(val) {});
myPromise.then(() => { doSomething(); });
myPromise.then((b) => { if (b) { return "yes" } else { forgotToReturn(); } });

param-names

Enforce standard parameter names for Promise constructors

Valid

new Promise(function (resolve) { ... })
new Promise(function (resolve, reject) { ... })

Invalid

new Promise(function (reject, resolve) { ... }) // incorrect order
new Promise(function (ok, fail) { ... }) // non-standard parameter names

Ensures that new Promise() is instantiated with the parameter names resolve, reject to avoid confusion with order such as reject, resolve. The Promise constructor uses the RevealingConstructor pattern. Using the same parameter names as the language specification makes code more uniform and easier to understand.

no-native

Ensure that Promise is included fresh in each file instead of relying on the existence of a native promise implementation. Helpful if you want to use bluebird or if you don't intend to use an ES6 Promise shim.

Valid

var Promise = require("bluebird");
var x = Promise.resolve("good");

Invalid

var x = Promise.resolve("bad");

Rule: no-return-wrap

Ensure that inside a then() or a catch() we always return or throw a raw value instead of wrapping in Promise.resolve or Promise.reject

Valid

myPromise.then(function(val) {
  return val * 2;
});
myPromise.then(function(val) {
  throw "bad thing";
});

Invalid

myPromise.then(function(val) {
  return Promise.resolve(val * 2);
});
myPromise.then(function(val) {
  return Promise.reject("bad thing");
})

Etc

• (c) MMXV jden [e-mail] - ISC license.
• (c) 2016 Jamund Ferguson [e-mail] - ISC license.