eslint-linter-browserify

Use eslint in the browser

import * as eslint from "eslint-linter-browserify";

// or const eslint = require("eslint-linter-browserify");
// or <script src="https://cdn.jsdelivr.net/npm/eslint-linter-browserify/linter.js"></script>
// or <script src="https://cdn.jsdelivr.net/npm/eslint-linter-browserify/linter.min.js"></script>

const linter = new eslint.Linter();

const messages = linter.verify("var foo;", {
  rules: {
    semi: ["error", "never"]
  }
}, { filename: "foo.js" });

console.log(messages);

https://eslint.org/docs/developer-guide/nodejs-api#linter

See the CodeMirror example for a way to use this in CodeMirror.