@typescript-eslint/no-misused-promises

A promise is an object, so it's always truthy, and if (isAdmin(user)) without await always takes the branch. An async function passed where a callback should return nothing is never awaited, so forEach moves on before the work is done, and nothing catches a rejection. A promise spread into an object adds no properties.

bun lint --type-aware turns this rule on in TypeScript files.

Configure#

Off by default. This rule needs types, so turning it on makes bun lint type check your code first.

bunfig.toml

[lint.rules]
"@typescript-eslint/no-misused-promises" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"@typescript-eslint/no-misused-promises" = ["error", {
  checksConditionals = true,
  checksVoidReturn = {
    arguments = true,
    attributes = false,
    inheritedMethods = true,
    properties = true,
    returns = true,
    variables = true,
  },
  checksSpreads = true,
}]

Examples#

example.ts3 problems

declare function isAdmin(id: string): Promise<boolean>;
declare function remove(id: string): Promise<void>;
declare function load(id: string): Promise<{ name: string }>;

function cleanUp(ids: string[]) {
  if (isAdmin(ids[0])) {
    ids.forEach(async id => {
      await remove(id);
    });
  }
  return { id: ids[0], ...load(ids[0]) };
}

example.tsNo problems

declare function isAdmin(id: string): Promise<boolean>;
declare function remove(id: string): Promise<void>;
declare function load(id: string): Promise<{ name: string }>;

async function cleanUp(ids: string[]) {
  if (await isAdmin(ids[0])) {
    for (const id of ids) {
      await remove(id);
    }
  }
  return { id: ids[0], ...(await load(ids[0])) };
}

An async callback in filter keeps every item, because the promise it returns is truthy:

example.ts1 problem

declare function isAdmin(id: string): Promise<boolean>;

const admins = ["ada", "lin"].filter(async id => await isAdmin(id));

Options#

Option	Default	What it does
`checksConditionals`	`true`	Report a promise as a condition: in `if`, `while`, `? :`, `!`, `&&`, `\|\|`, and returned from the callback of `filter`, `find`, `some` and the like.
`checksVoidReturn`	`{ "attributes": false }`	Report a function that returns a promise where a function that returns `void` is expected.
`checksSpreads`	`true`	Report `...` in front of a promise.

checksVoidReturn is true, false, or an object that turns the check off in some places. A key you leave out is true.

JSX attributes aren't checked by default, because <form onSubmit={save}> with an async function is how most React code handles events. With an ESLint config file the default is typescript-eslint's, true.

Key	Where the function is
`arguments`	An argument of a call or of `new`.
`attributes`	A JSX attribute.
`inheritedMethods`	A method, when the class or interface it extends or implements declares it as returning `void`.
`properties`	A property of an object.
`returns`	A value that a function returns.
`variables`	A value assigned to a variable.

Not reported with checksVoidReturn: { arguments: false }:

example.tswith { "checksVoidReturn": { "arguments": false } }No problems

declare function remove(id: string): Promise<void>;

function cleanUp(ids: string[]) {
  ids.forEach(async id => {
    await remove(id);
  });
}

Not reported by default, and reported with checksVoidReturn: true:

example.tsxNo problems

declare namespace JSX {
  interface IntrinsicElements {
    form: { onSubmit?: () => void };
  }
}
declare function save(): Promise<void>;

const form = <form onSubmit={save} />;

example.tsxwith { "checksVoidReturn": true }1 problem

declare namespace JSX {
  interface IntrinsicElements {
    form: { onSubmit?: () => void };
  }
}
declare function save(): Promise<void>;

const form = <form onSubmit={save} />;

checksConditionals also takes { "flagUnions": .. }, for a condition that's only sometimes a promise. With "none" (default) it's allowed. With "all" it's reported. With "strict" it's reported if the union has both the promise and what it resolves to, like number | Promise<number>.

Not reported by default:

example.tsNo problems

declare function cached(key: string): number | Promise<number>;

if (cached("total")) {
  console.log("found");
}

Reported with checksConditionals: { flagUnions: "strict" }:

example.tswith { "checksConditionals": { "flagUnions": "strict" } }1 problem

declare function cached(key: string): number | Promise<number>;

if (cached("total")) {
  console.log("found");
}