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noFloatingPromises

Summary

Section titled Summary

Description

Section titled Description

Require Promise-like statements to be handled appropriately.

A “floating” Promise is one that is created without any code set up to handle any errors it might throw. Floating Promises can lead to several issues, including improperly sequenced operations, unhandled Promise rejections, and other unintended consequences.

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

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

Examples

Section titled Examples

Invalid

Section titled Invalid
async function returnsPromise(): Promise<string> {
  return 'value';
}
returnsPromise().then(() => {});
code-block.ts:4:1 lint/nursery/noFloatingPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
    2 │   return ‘value’;
    3 │ }
  > 4 │ returnsPromise().then(() => {});
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    5 │ 
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
const returnsPromise = async (): Promise<string> => {
  return 'value';
}
async function returnsPromiseInAsyncFunction() {
  returnsPromise().then(() => {});
}
code-block.ts:5:3 lint/nursery/noFloatingPromises  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
    3 │ }
    4 │ async function returnsPromiseInAsyncFunction() {
  > 5 │   returnsPromise().then(() => {});
      │   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    6 │ }
    7 │ 
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
  ℹ Unsafe fix: Add await operator.
  
    5 │ ··await·returnsPromise().then(()·=>·{});
      │   ++++++                                
const promise = new Promise((resolve) => resolve('value'));
promise.then(() => { }).finally(() => { });
code-block.ts:2:1 lint/nursery/noFloatingPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
    1 │ const promise = new Promise((resolve) => resolve(‘value’));
  > 2 │ promise.then(() => { }).finally(() => { });
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
Promise.all([p1, p2, p3])
code-block.ts:1:1 lint/nursery/noFloatingPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
  > 1 │ Promise.all([p1, p2, p3])
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
class Api {
  async returnsPromise(): Promise<string> {
    return 'value';
  }
  async someMethod() {
    this.returnsPromise();
  }
}
code-block.ts:6:5 lint/nursery/noFloatingPromises  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
    4 │   }
    5 │   async someMethod() {
  > 6 │     this.returnsPromise();
      │     ^^^^^^^^^^^^^^^^^^^^^^
    7 │   }
    8 │ }
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
  ℹ Unsafe fix: Add await operator.
  
    6 │ ····await·this.returnsPromise();
      │     ++++++                      
class Parent {
  async returnsPromise(): Promise<string> {
    return 'value';
  }
}


class Child extends Parent {
  async someMethod() {
    this.returnsPromise();
  }
}
code-block.ts:9:5 lint/nursery/noFloatingPromises  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
     7 │ class Child extends Parent {
     8 │   async someMethod() {
   > 9 │     this.returnsPromise();
       │     ^^^^^^^^^^^^^^^^^^^^^^
    10 │   }
    11 │ }
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
  ℹ Unsafe fix: Add await operator.
  
    9 │ ····await·this.returnsPromise();
      │     ++++++                      
class Api {
  async returnsPromise(): Promise<string> {
    return 'value';
  }
}
const api = new Api();
api.returnsPromise().then(() => {}).finally(() => {});
code-block.ts:7:1 lint/nursery/noFloatingPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
    5 │ }
    6 │ const api = new Api();
  > 7 │ api.returnsPromise().then(() => {}).finally(() => {});
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    8 │ 
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
const obj = {
  async returnsPromise(): Promise<string> {
    return 'value';
  },
};


obj.returnsPromise();
code-block.ts:7:1 lint/nursery/noFloatingPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
    5 │ };
    6 │ 
  > 7 │ obj.returnsPromise();
      │ ^^^^^^^^^^^^^^^^^^^^^
    8 │ 
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
type Props = {
  returnsPromise: () => Promise<void>;
};


async function testCallingReturnsPromise(props: Props) {
  props.returnsPromise();
}
code-block.ts:6:3 lint/nursery/noFloatingPromises  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A “floating” Promise was found, meaning it is not properly handled and could lead to ignored errors or unexpected behavior.
  
    5 │ async function testCallingReturnsPromise(props: Props) {
  > 6 │   props.returnsPromise();
      │   ^^^^^^^^^^^^^^^^^^^^^^^
    7 │ }
    8 │ 
  
  ℹ This happens when a Promise is not awaited, lacks a .catch or .then rejection handler, or is not explicitly ignored using the void operator.
  
  ℹ Unsafe fix: Add await operator.
  
    6 │ ··await·props.returnsPromise();
      │   ++++++

Valid

Section titled Valid
async function returnsPromise(): Promise<string> {
  return 'value';
}


await returnsPromise();


void returnsPromise();


// Calling .then() with two arguments
returnsPromise().then(
  () => {},
  () => {},
);


// Calling .catch() with one argument
returnsPromise().catch(() => {});


await Promise.all([p1, p2, p3])


class Api {
  async returnsPromise(): Promise<string> {
    return 'value';
  }
  async someMethod() {
    await this.returnsPromise();
  }
}
type Props = {
  returnsPromise: () => Promise<void>;
};


async function testCallingReturnsPromise(props: Props) {
  return props.returnsPromise();
}

How to configure

Section titled How to configure
biome.json
{
  "linter": {
    "rules": {
      "nursery": {
        "noFloatingPromises": "error"
      }
    }
  }
}
Section titled Related links