prefer-ascii

Ensures that the code is fully written in ASCII characters.

V8, the JavaScript engine Deno relies on, provides a method that strings get populated outside V8's heap. In particular, if they are composed of one-byte characters only, V8 can handle them much more efficiently through v8::String::ExternalOneByteStringResource. In order to leverage this V8 feature in the internal of Deno, this rule checks if all characters in the code are ASCII.

That said, you can also make use of this lint rule for something other than Deno's internal JavaScript code. If you want to make sure your codebase is made up of ASCII characters only (e.g. want to disallow non-ASCII identifiers) for some reasons, then this rule will be helpful.

Invalid:

const π = Math.PI;

// string literals are also checked
const ninja = "🥷";

function こんにちは(名前: string) {
  console.log(`こんにちは、${名前}さん`);
}

// “comments” are also checked
// ^        ^
// |        U+201D
// U+201C

Valid:

const pi = Math.PI;

const ninja = "ninja";

function hello(name: string) {
  console.log(`Hello, ${name}`);
}

// "comments" are also checked