@typescript-eslint/no-unnecessary-type-assertion

An assertion tells the compiler "trust me". If the expression already has that type, or the place that receives it accepts the original type, the assertion does nothing today. But it stays in the code and silences the compiler when the types change later.

--fix removes the assertion.

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-unnecessary-type-assertion" = "error" # or "warn"

Every option, set to its default:

bunfig.toml

[lint.rules]
"@typescript-eslint/no-unnecessary-type-assertion" = ["error", {
  checkLiteralConstAssertions = false,
  typesToIgnore = [], # list of strings
}]

Examples#

example.ts3 problems

interface User {
  name: string;
  email?: string;
}

function show(user: User) {
  const name = user.name as string;
  const length = user.name!.length;
  const email: string | undefined = user.email!;
  return [name, length, email];
}

example.tsNo problems

interface User {
  name: string;
  email?: string;
}

function show(user: User) {
  const name = user.name;
  const length = user.name.length;
  const email: string = user.email!;
  return [name, length, email];
}

An assertion that narrows or widens the type isn't reported:

example.tsNo problems

function parse(text: string) {
  const data = JSON.parse(text) as { id: number };
  let mode = "dark" as "dark" | "light";
  return [data, mode];
}

Options#

Option	Default	What it does
`checkLiteralConstAssertions`	`false`	Also report `as const` on a literal in a `const` declaration, where the type is already the literal type.
`typesToIgnore`	`[]`	Type names, as written after `as`. Assertions to these types are never reported.

Reported only with checkLiteralConstAssertions:

example.tswith { "checkLiteralConstAssertions": true }1 problem

const retries = 3 as const;

Not reported with "typesToIgnore": ["UserId"]:

example.tswith { "typesToIgnore": ["UserId"] }No problems

type UserId = string;

function key(id: UserId) {
  return id as UserId;
}