always-return.md

Require returning inside each then() to create readable and reusable Promise chains (promise/always-return)

💼 This rule is enabled in the following configs: ✅ flat/recommended, ✅ recommended.

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()
  }
})

Options

ignoreLastCallback

You can pass an { ignoreLastCallback: true } as an option to this rule so that the last then() callback in a promise chain does not warn if it does not have a return. Default is false.

// OK
promise.then((x) => {
  console.log(x)
})
// OK
void promise.then((x) => {
  console.log(x)
})
// OK
await promise.then((x) => {
  console.log(x)
})

promise
  // NG
  .then((x) => {
    console.log(x)
  })
  // OK
  .then((x) => {
    console.log(x)
  })

// NG
const v = promise.then((x) => {
  console.log(x)
})
// NG
const v = await promise.then((x) => {
  console.log(x)
})
function foo() {
  // NG
  return promise.then((x) => {
    console.log(x)
  })
}

ignoreAssignmentVariable

You can pass an { ignoreAssignmentVariable: [] } as an option to this rule with a list of variable names so that the last then() callback in a promise chain does not warn if it does an assignment to a global variable. Default is ["globalThis"].

/* eslint promise/always-return: ["error", { ignoreAssignmentVariable: ["globalThis"] }] */

// OK
promise.then((x) => {
  globalThis = x
})

promise.then((x) => {
  globalThis.x = x
})

// OK
promise.then((x) => {
  globalThis.x.y = x
})

// NG
promise.then((x) => {
  anyOtherVariable = x
})

// NG
promise.then((x) => {
  anyOtherVariable.x = x
})

// NG
promise.then((x) => {
  x()
})