index.js

const fs = require('fs');

function aadecode(text) {
    var evalPreamble = "(\uFF9F\u0414\uFF9F) ['_'] ( (\uFF9F\u0414\uFF9F) ['_'] (";
    var decodePreamble = "( (\uFF9F\u0414\uFF9F) ['_'] (";
    var evalPostamble = ") (\uFF9F\u0398\uFF9F)) ('_');";
    var decodePostamble = ") ());";

    text = text.replace(/^\s*/, "").replace(/\s*$/, "");

    if (/^\s*$/.test(text)) {
        return "";
    }

    if (text.lastIndexOf(evalPreamble) < 0) {
        throw new Error("Given code is not encoded as aaencode.");
    }
    if (text.lastIndexOf(evalPostamble) != text.length - evalPostamble.length) {
        throw new Error("Given code is not encoded as aaencode.");
    }

    var decodingScript = text.replace(evalPreamble, decodePreamble)
                             .replace(evalPostamble, decodePostamble);
    return eval(decodingScript);
}

var input = fs.readFileSync('input.js', 'utf8');

var output = aadecode(input);
fs.writeFileSync('output.js', output, 'utf8');