Why is Gulp-Uglify's keep_fnames setting not working?

Question

JavaScript

It is necessary that gulp-uglify does not minimize function names during minification (variable names need to be minimized), in order to be able to get the class name in es6 (babel) through this.constructor.name. With minification gives the name of the minified function - 'e'.
I set the keep_fnames parameter, but uglify successfully ignores it, and produces the same code, both with true and false.

.pipe(uglify({
       compress: {
           'keep_fnames': true
       }
   }))

What am I doing wrong?
PS: if you set mangle: false, then neither function names nor variables are minified, which does not suit me, variables should be minified.

Answer 1

Try like this:

.pipe(uglify({
    mangle: {
        keep_fnames: true
    }
}))