Is the underscore prefix for property and method names merely a convention?

Is the underscore prefix in JavaScript only a convention, like for example in Python private class methods are?

From the 2.7 Python documentation:

“Private” instance variables that
cannot be accessed except from inside
an object don’t exist in Python.
However, there is a convention that is
followed by most Python code: a name
prefixed with an underscore (e.g.
_spam) should be treated as a non-public part of the API (whether it
is a function, a method or a data
member).

Does this also apply to JavaScript?

Take for example this JavaScript code:

function AltTabPopup() {
    this._init();
}

AltTabPopup.prototype = {
    _init : function() {
        ...
    }
}

Also, underscore prefixed variables are used.

    ...
    this._currentApp = 0;
    this._currentWindow = -1;
    this._thumbnailTimeoutId = 0;
    this._motionTimeoutId = 0;
    ...

Only conventions? Or is there more behind the underscore prefix?

I admit my question is quite similar to this question, but it didn’t make one smarter about the significance of the underscore prefix in JavaScript.