await-promise.md

Disallows awaiting a value that is not a Promise (await-promise)

This rule disallows awaiting a value that is not a Promise. While it is valid JavaScript to await a non-Promise-like value (it will resolve immediately), this pattern is often a programmer error, such as forgetting to add parenthesis to call a function that returns a Promise.

Rule Details

Examples of incorrect code for this rule:

await 'value';

const createValue = () => 'value';
await createValue();

// An array of Promises is not the same as an AsyncIterable
async function incorrect(arrayOfPromises: Array<Promise<string>>) {
  for await (const element of arrayOfPromises) {
  }
}

Examples of correct code for this rule:

await Promise.resolve('value');

const createValue = (async() = 'value');
await createValue();

async function overIterable(iterable: AsyncIterable<string>) {
  for await (const element of iterable) {
  }
}

async function overIterableIterator(iterable: AsyncIterableIterator<string>) {
  for await (const element of iterable) {
  }
}

Options

The rule accepts an options object with the following property:

• allowedPromiseNames any extra names of classes or interfaces to be considered "awaitable" in await statements.

Classes named Promise may always be awaited. allowedPromiseNames does not affect for-await-of statements.

allowedPromiseNames

Examples of incorrect code for this rule with { allowedPromiseNames: ["Thenable"] }:

class Thenable {
  /* ... */
}

await new Thenable();

Examples of correct code for this rule with { allowedPromiseNames: ["Thenable"] }:

class OtherClass {
  /* ... */
}

await new OtherClass();

When Not To Use It

If you want to allow code to await non-Promise values. This is generally not preferred, but can sometimes be useful for visual consistency.

Related to

• TSLint: 'await-promise'