no-floating-promises.md

description
Require Promise-like statements to be handled appropriately.

🛑 This file is source code, not the primary documentation location! 🛑

See https://typescript-eslint.io/rules/no-floating-promises for documentation.

A "floating" Promise is one that is created without any code set up to handle any errors it might throw. Floating Promises can cause several issues, such as improperly sequenced operations, ignored Promise rejections, and more.

This rule reports when a Promise is created and not properly handled. Valid ways of handling a Promise-valued statement include:

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

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()

:::tip no-floating-promises only detects unhandled Promise statements. See no-misused-promises for detecting code that provides Promises to logical locations such as if statements. :::

Examples

❌ Incorrect

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);

✅ Correct

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

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

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

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

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

Options

ignoreVoid

This allows you to stop the rule reporting promises consumed with void operator. This can be a good way to explicitly mark a promise as intentionally not awaited.

Examples of correct code for this rule with { ignoreVoid: true }:

async function returnsPromise() {
  return 'value';
}
void returnsPromise();

void Promise.reject('value');

With this option set to true, and if you are using no-void, you should turn on the allowAsStatement option.

ignoreIIFE

This allows you to skip checking of async IIFEs (Immediately Invoked function Expressions).

Examples of correct code for this rule with { ignoreIIFE: true }:

await(async function () {
  await res(1);
})();

(async function () {
  await res(1);
})();

When Not To Use It

This rule can be difficult to enable on large existing projects that set up many floating Promises. Alternately, if you're not worried about crashes from floating or misused Promises -such as if you have global unhandled Promise handlers registered- then in some cases it may be safe to not use this rule. You might consider using voids and/or ESLint disable comments for those specific situations instead of completely disabling this rule.

Related To

• no-misused-promises

Further Reading

• "Using Promises" MDN documentation. Note especially the sections on Promise rejection events and Composition.