typescript/no-unnecessary-type-assertion Suspicious

🚧 An auto-fix is still under development.

What it does

This rule disallows type assertions that do not change the type of an expression.

Why is this bad?

Type assertions that don't actually change the type of an expression are unnecessary and can be safely removed. They add visual noise without providing any benefit and may indicate confusion about TypeScript's type system.

Examples

Examples of incorrect code for this rule:

const str: string = "hello";
const redundant = str as string; // unnecessary, str is already string

function getString(): string {
  return "hello";
}
const result = getString() as string; // unnecessary, getString() already returns string

const num = 42;
const alsoRedundant = num as 42; // unnecessary if TypeScript can infer literal type

// Unnecessary assertion to wider type
const literal = "hello" as string;

Examples of correct code for this rule:

const unknown: unknown = "hello";
const str = unknown as string; // necessary to narrow type

const element = document.getElementById("myElement") as HTMLInputElement; // necessary for specific element type

const obj = { name: "John" };
const name = obj.name as const; // necessary for literal type

// No assertion needed
const str2: string = "hello";
const num: number = 42;

How to use

To enable this rule in the CLI or using the config file, you can use:

CLI

oxlint --deny typescript/no-unnecessary-type-assertion

Config (.oxlintrc.json)

{
  "rules": {
    "typescript/no-unnecessary-type-assertion": "error"
  }
}

References

• Rule Source