promise/always-return Suspicious

What it does

Require returning inside each then() to create readable and reusable Promise chains. We also allow someone to throw inside a then() which is essentially the same as return Promise.reject().

Why is this bad?

Broken Promise Chain. Inside the first then() callback, a function is called but not returned. This causes the next then() in the chain to execute immediately without waiting for the called function to complete.

Examples

Examples of incorrect code for this rule:

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

Examples of correct code for this rule:

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";
  }
});

Configuration

This rule accepts a configuration object with the following properties:

ignoreAssignmentVariable

type: string[]

default: ["globalThis"]

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();
});

ignoreLastCallback

type: boolean

default: false

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);
  });
}

How to use

To enable this rule using the config file or in the CLI, you can use:

Config (.oxlintrc.json)

{
  "plugins": ["promise"],
  "rules": {
    "promise/always-return": "error"
  }
}

CLI

oxlint --deny promise/always-return --promise-plugin

References

• Rule Source