typescript/no-floating-promises Correctness

What it does

This rule disallows "floating" Promises in TypeScript code, which is a Promise that is created without any code to handle its resolution or rejection.

This rule will report Promise-valued statements that are not treated in one of the following ways:

• Calling its .then() with two arguments
• Calling its .catch() with one argument
• awaiting it
• returning it
• voiding it

This rule also reports when an Array containing Promises is created and not properly handled. The main way to resolve this is by using one of the Promise concurrency methods to create a single Promise, then handling that according to the procedure above. These methods include:

• Promise.all()
• Promise.allSettled()
• Promise.any()
• Promise.race()

Why is this bad?

Floating Promises can cause several issues, such as improperly sequenced operations, ignored Promise rejections, and more.

Examples

Examples of incorrect code for this rule:

const promise = new Promise((resolve, reject) => resolve("value"));
promise;

async function returnsPromise() {
  return "value";
}
returnsPromise().then(() => {});

Promise.reject("value").catch();

Promise.reject("value").finally();

[1, 2, 3].map(async (x) => x + 1);

Examples of correct code for this rule:

const promise = new Promise((resolve, reject) => resolve("value"));
await promise;

async function returnsPromise() {
  return "value";
}

void returnsPromise();

returnsPromise().then(
  () => {},
  () => {},
);

Promise.reject("value").catch(() => {});

await Promise.reject("value").finally(() => {});

await Promise.all([1, 2, 3].map(async (x) => x + 1));

Configuration

This rule accepts a configuration object with the following properties:

allowForKnownSafeCalls

type: array

default: []

Allows specific calls to be ignored, specified as type or value specifiers.

allowForKnownSafeCalls[n]

type: string

Type or value specifier for matching specific declarations

Supports four types of specifiers:

1. String specifier (deprecated): Universal match by name

"Promise"

2. File specifier: Match types/values declared in local files

{ "from": "file", "name": "MyType" }
{ "from": "file", "name": ["Type1", "Type2"] }
{ "from": "file", "name": "MyType", "path": "./types.ts" }

3. Lib specifier: Match TypeScript built-in lib types

{ "from": "lib", "name": "Promise" }
{ "from": "lib", "name": ["Promise", "PromiseLike"] }

4. Package specifier: Match types/values from npm packages

{ "from": "package", "name": "Observable", "package": "rxjs" }
{ "from": "package", "name": ["Observable", "Subject"], "package": "rxjs" }

allowForKnownSafePromises

type: array

default: []

Allows specific Promise types to be ignored, specified as type or value specifiers.

allowForKnownSafePromises[n]

type: string

Type or value specifier for matching specific declarations

Supports four types of specifiers:

1. String specifier (deprecated): Universal match by name

"Promise"

2. File specifier: Match types/values declared in local files

{ "from": "file", "name": "MyType" }
{ "from": "file", "name": ["Type1", "Type2"] }
{ "from": "file", "name": "MyType", "path": "./types.ts" }

3. Lib specifier: Match TypeScript built-in lib types

{ "from": "lib", "name": "Promise" }
{ "from": "lib", "name": ["Promise", "PromiseLike"] }

4. Package specifier: Match types/values from npm packages

{ "from": "package", "name": "Observable", "package": "rxjs" }
{ "from": "package", "name": ["Observable", "Subject"], "package": "rxjs" }

checkThenables

type: boolean

default: false

Check for thenable objects that are not necessarily Promises.

ignoreIIFE

type: boolean

default: false

Ignore immediately invoked function expressions (IIFEs).

ignoreVoid

type: boolean

default: true

Ignore Promises that are void expressions.

How to use

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

Config (.oxlintrc.json)

{
  "rules": {
    "typescript/no-floating-promises": "error"
  }
}

CLI

oxlint --type-aware --deny typescript/no-floating-promises

References

• Rule Source
• Rule Source (tsgolint)