Blocking usage of the any type in TypeScript codebases

The any type in TypeScript denotes that the value can be anything - string, number, boolean etc. If widely used in a new codebase, it would defeat the purpose of using TypeScript over untyped JavaScript. So it’s preferable to block the usage of this type. If you’re incrementally migrating a JavaScript codebase to TypeScript, it would be better to use the new unknown type.

Blocking implicit any

function main() {
frobulate("hello");
}

function frobulate(name) {
console.log(name);
}

The name argument in the above frobulate function has an implicit any type. Usually, TypeScript can infer types based on the control flow, assignments etc but in the above case, it’s hard to infer the type for the name argument so it would implicitly have the any type.

To block the implicit any, we can set noImplicitAny or strict to true in tsconfig.json.

Blocking explicit any

Setting noImplicitAny or strict to true would throw errors for the above code but if you explicitly type the name argument as any, it wouldn’t throw any errors.

function main() {
frobulate("hello");
}

// Notice the "any" type added to the "name" argument
function frobulate(name: any) {
console.log(name);
}

To block explicit any, we need to use ESLint with the typescript-eslint plugin and turn on the no-explicit-any rule. Here’s an example .eslintrc config:

{
"parser": "@typescript-eslint/parser",
"parserOptions": {
"sourceType": "module"
},
"plugins": ["@typescript-eslint"],
"rules": {
"@typescript-eslint/no-explicit-any": ["error"]
}
}