@typescript-eslint/no-unsafe-assignment

When you assign an any to a variable, the variable becomes any too, or it claims a type that nobody checked. From there, every property read, call and argument goes unchecked as well, far from where the any came in.

The rule checks variable declarations, assignments, default values, class properties, object properties, JSX attributes, destructuring and array spreads. It also reports any inside a generic type, such as a Set<any> assigned to a Set<string>.

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.

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

Examples#

const config = JSON.parse("{}");
const port: number = JSON.parse("8080");
const { host } = JSON.parse("{}") as { host: any };
const tags: Set<string> = new Set<any>();

interface Config {
  host: string;
  port: number;
}

const config = JSON.parse("{}") as Config;
const { host } = config;
const tags: Set<string> = new Set<string>();

Assigning an any to something declared as unknown is allowed. That's the safe way to receive a value you know nothing about:

const data: unknown = JSON.parse("{}");

if (typeof data === "string") {
  console.log(data.length);
}

When not to use it#

In a codebase that's still moving from JavaScript to TypeScript, any is everywhere and the reports drown out everything else. Turn the rule on once most values have real types.