How do you know if the SharePoint page is in Edit Mode from JavaScript?

Is there a way to find out if a SharePoint page is in Edit Mode from JavaScript?
Yes there is.

Look for the MSOLayout_InDesignMode attribute in the javascript.

var inDesignMode = document.forms[MSOWebPartPageFormName].MSOLayout_InDesignMode.value;

if (inDesignMode == "1")
{
    // page is in edit mode
}
else
{
    // page is in browse mode
}

This will refer to value of the following html input control, which is rendering on the page when it is in edit mode:

For wiki pages, you will need _wikiPageMode parameter

var wikiInEditMode = document.forms[MSOWebPartPageFormName]._wikiPageMode.value;
if (wikiInEditMode == "Edit")
{
    // wiki page is in edit mode
}
else
{
    // wiki page is not in edit mode
}