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:
javascript
myPromise.then(function (val) {})
myPromise.then(() => {
doSomething()
})
myPromise.then((b) => {
if (b) {
return 'yes'
} else {
forgotToReturn()
}
})Examples of correct code for this rule:
javascript
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"].
javascript
/* 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.
javascript
// 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:
json
{
"plugins": ["promise"],
"rules": {
"promise/always-return": "error"
}
}bash
oxlint --deny promise/always-return --promise-plugin