@typescript-eslint/prefer-as-const

"dark" as "dark" and let theme: "dark" = "dark" write the same literal twice, and you need to change both every time. as const gives you the same narrow type and takes it from the value.

The rule checks every as assertion, and the type annotations on variables and class fields. It only reports a type that's the same literal as the value.

--fix rewrites "dark" as "dark" to "dark" as const. A type annotation is reported but not fixed.

This rule is on by default in TypeScript files.

Configure#

On by default as a warning. To turn it off:

[lint.rules]
"@typescript-eslint/prefer-as-const" = "off" # or "error"

Examples#

let theme = "dark" as "dark";
let retries: 3 = 3;

class Request {
  method: "GET" = "GET";
}

let theme = "dark" as const;
let retries = 3 as const;

class Request {
  method = "GET" as const;
}

let mode: "dark" | "light" = "dark";
let size = "small" as "small" | "large";