I believe the answer you are looking for is linked in the accepted answer to the question you linked: http://www.jslint.com/help.html#forin

does not recommend use of the for in statement. Use Object.keys instead.

It goes on to explain why that is recommended.

The for in statement allows for looping through the names of all of the properties of an object. Unfortunately, it also loops through all of the properties that were inherited through the prototype chain. This has the bad side effect of serving up method functions when the interest is in data properties. If a program is written without awareness of this situation, then it can fail.

The body of every for in statement should be wrapped in an if statement that does filtering. It can select for a particular type or range of values, or it can exclude functions, or it can exclude properties from the prototype. For example,

for (name in object) {
    if (object.hasOwnProperty(name)) {
        ....
    }
}

Note that the above code will fail if the object contains a data property named hasOwnProperty. Use Object.keys instead.

In modern (ES2015) JavaScript you can use Array.from():

Array.from(arguments).forEach((arg) => {
  this.score += arg;
});

You don't need to use array indexing, as the .forEach() function passes each array element to the callback function as the first argument. (It does pass the index as the second argument, but in this case you wouldn't need to use that.)

It's important to note that if the arguments object is passed out of a function (as it would be here), the overall function may be considered ineligible for optimization. That's because the arguments object has some weird properties that make it very hard for the optimizer to know what's going on and whether it's safe to make assumptions about how local variables change. If that's a concern, then the only option is to copy the arguments into a simple array with a for loop.

In anything except old IE (IE8 and older), you can do this:

["foo","bar","blah"].forEach(function(key) {
    // do something
});

To add support in some versions of IE (I think IE7 and 8 allow this, IE6 does not):

if( ![].forEach) {
    Array.prototype.forEach = function(callback) {
        for( var i=0, l=this.length; i<l; i++) callback(this[i]);
    };
}

Just a simple for loop should be quicker if you need to loop.

var l = collection.length;
for (var i = 0; i<l; i++) {
  //do stuff
} 

But, just because it's quicker doesn't mean it's always important that it is so.

This runs at the client, not the server, so you don't need to worry about scaling with the number of users, and if it's quick with a .each(), then leave it. But, if that's slow, a for loop could speed it up.