Unable to minify Vue.js application

Those compressors simply only support an old version of JavaScript. Their support is restricted to at most ES5. To make your code work, convert it:

(function() {
    initApp();
})();

function initApp() {
    window.myApp = new Vue({
        el: '#wrapper',
        data: function() { // changed this line
            return {
                somedata: []
            }
        }
    });
}

And it should compress.

Details:

They use uglify-js: "^3.3.10", that is known for not supporting ES6(uglify-es does) .

From their repo (emphasis mine):

UglifyJS 3

UglifyJS is a JavaScript parser, minifier, compressor and beautifier toolkit.

Note:

• (...)
• uglify-js only supports JavaScript (ECMAScript 5).
• To minify ECMAScript 2015 or above, transpile using tools like Babel.