noMisusedPromises

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TypeScript and TSX

Summary

Rule available since: v2.1.0
Diagnostic Category: lint/nursery/noMisusedPromises
This rule has an unsafe fix.
The default severity of this rule is information.
This rule belongs to the following domains:
- project
Sources:
- Same as @typescript-eslint/no-misused-promises

How to configure

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{
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  "linter": {
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    "rules": {
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      "nursery": {
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        "noMisusedPromises": "error"
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      }
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    }
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  }
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}

Description

Disallow Promises to be used in places where they are almost certainly a mistake.

In most cases, if you assign a Promise somewhere a Promise is not allowed, the TypeScript compiler will be able to catch such a mistake. But there are a few places where TypeScript allows them — they’re not necessarily a mistake — even though they could be considered almost certainly to be one.

This rule disallows using Promises in such places.

Examples

Invalid

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const promise = Promise.resolve('value');
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if (promise) { /* This branch will always execute */ }

/promise-in-condition.js:2:5 lint/nursery/noMisusedPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A Promise was found where a conditional was expected.
  
    1 │ const promise = Promise.resolve(‘value’);
  > 2 │ if (promise) { /* This branch will always execute */ }
      │     ^^^^^^^
    3 │ 
  
  ℹ A Promise is always truthy, so this is most likely a mistake.
  
  ℹ You may have intended to await the Promise instead.

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const promise = Promise.resolve('value');
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const val = promise ? 123 : 456; // Always evaluates to `123`.

/promise-in-ternary-condition.js:2:13 lint/nursery/noMisusedPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A Promise was found where a conditional was expected.
  
    1 │ const promise = Promise.resolve(‘value’);
  > 2 │ const val = promise ? 123 : 456; // Always evaluates to 123.
      │             ^^^^^^^
    3 │ 
  
  ℹ A Promise is always truthy, so this is most likely a mistake.
  
  ℹ You may have intended to await the Promise instead.

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// The following filter has no effect:
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const promise = Promise.resolve('value');
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[1, 2, 3].filter(() => promise);

/promise-in-filter.js:3:18 lint/nursery/noMisusedPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This function returns a Promise where a conditional was expected.
  
    1 │ // The following filter has no effect:
    2 │ const promise = Promise.resolve(‘value’);
  > 3 │ [1, 2, 3].filter(() => promise);
      │                  ^^^^^^^^^^^^^
    4 │ 
  
  ℹ A Promise is always truthy, so this is most likely a mistake.
  
  ℹ You may have intended to await the Promise, but this does not work inside a synchronous callback.

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const promise = Promise.resolve('value');
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while (promise) { /* This is an endless loop */ }

/promise-while-condition.js:2:8 lint/nursery/noMisusedPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A Promise was found where a conditional was expected.
  
    1 │ const promise = Promise.resolve(‘value’);
  > 2 │ while (promise) { /* This is an endless loop */ }
      │        ^^^^^^^
    3 │ 
  
  ℹ A Promise is always truthy, so this is most likely a mistake.
  
  ℹ You may have intended to await the Promise instead.

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// Using a `Promise` as an iterable expands to nothing:
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const getData = () => fetch('/');
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console.log({ foo: 42, ...getData() });

/spread-promise.js:3:27 lint/nursery/noMisusedPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ A Promise was found where an iterable was expected.
  
    1 │ // Using a Promise as an iterable expands to nothing:
    2 │ const getData = () => fetch(’/’);
  > 3 │ console.log({ foo: 42, …getData() });
      │                           ^^^^^^^^^
    4 │ 
  
  ℹ The spread syntax is used to expand an iterable, but a Promise needs to be await-ed to take its value.

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// These `fetch`-es are not `await`-ed in order:
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[1, 2, 3].forEach(async value => {
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    await fetch(`/${value}`);
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});

/promise-in-forEach.js:2:19 lint/nursery/noMisusedPromises ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ This function returns a Promise, but no return value was expected.
  
    1 │ // These fetch-es are not await-ed in order:
  > 2 │ [1, 2, 3].forEach(async value => {
      │                   ^^^^^^^^^^^^^^^^
  > 3 │     await fetch(/$&#123;value&#125;);
  > 4 │ });
      │ ^
    5 │ 
  
  ℹ This may not have the desired result if you expect the Promise to be await-ed.

Valid

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const promise = Promise.resolve('value');
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if (await promise) { /* Do something */ }
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const val = (await promise) ? 123 : 456;
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while (await promise) { /* Do something */ }
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const getData = () => fetch('/');
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console.log({ foo: 42, ...(await getData()) });
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// for-of puts `await` in outer context:
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for (const value of [1, 2, 3]) {
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    await doSomething(value);
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}