mangle-props and Object.defineProperty

[philipbulley]

The following is pre-uglified:

var d = Date.prototype;
Object.defineProperty(d, "year", {
  get: function () {
    return this.getFullYear()
  },
  set: function (y) {
    this.setFullYear(y)
  }
});

function demo() {
  var now = new Date;
  console.log(now.year); // 2000
  now.year = 2001; // 987617605170
  console.log(now);  // Wed Apr 18 11:13:25 GMT-0700 (Pacific Daylight Time) 2001
}

demo();

Which is uglified using:

uglifyjs --compress --beautify --mangle --mangle-props test.js

And produces:

function demo() {
    var e = new Date();
    console.log(e.c), e.c = 2001, console.log(e);
}

var d = Date.prototype;

Object.defineProperty(d, "year", {
    a: function() {
        return this.getFullYear();
    },
    b: function(e) {
        this.setFullYear(e);
    }
}), demo();

The now.year getter/setter has become e.c, but the string "year" in Object.defineProperty() remains. Surely "year" should become "c"?

[fabiosantoscode]

This is going to be a tough one to fix. defineProperty creates a property and takes a string. That string needs to be changed, but uglifyjs doesn't always know how to mangle the string.

Another point is that the get and set names shouldn't be mangled either.

What I think of this second point is that this really shows why you really need to use --mangle-props together with a regexp. There are lots of APIs which won't take these new custom objects. Still needs a fix though.

[rvanvelzen]

This is why closure compiler never changes x["foo"] to x.foo - it allows the author to do this kind of stuff.

Generally, when using defineProperty, you should either not mangle props or specify a regex.

We could try to do sneaky stuff like automatically adding the second argument of a defineProperty call to the excluded props, but that seems way too magic-y.

[philipbulley]

That's pretty much what I've ended up doing. In case it helps anyone in future, I first whitelist a bunch of properties as an array. Then blacklist/remove from that array any properties that have been implemented using defineProperty:

js = fs.readFileSync('main.js', 'utf8');
regex = /Object.defineProperty\(\s?.+?,\s?"(.+?)"/g
while(match = regex.exec(js)){
    // Ensure match[1] won't appear in my mangle regex
}

Note: That particular regex works well with the JS output from the Typescript compiler.