Enforce best practices for JavaScript promises.

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.

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": "error",
        "promise/no-return-wrap": "error",
        "promise/param-names": "error",
        "promise/catch-or-return": "error",
        "promise/no-native": "off",
        "promise/no-nesting": "warn",
        "promise/no-promise-in-callback": "warn",
        "promise/no-callback-in-promise": "warn",
        "promise/avoid-new": "warn"
    }
}

or start with the recommended rule set

{
    "extends": [
        "plugin:promise/recommended"
    ]
}

Key

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.

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

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

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.

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)

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().

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" } });

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

Enforce standard parameter names for Promise constructors

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

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.

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.

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

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

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

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

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

• (c) MMXV jden [email protected] - ISC license.
• (c) 2016 Jamund Ferguson [email protected] - ISC license.