Detect Edit Mode with Javascript on SharePoint pages or SPListitem forms

This JavaScript function will tell you if we are in the List Edit form:

var isEditForm = function ()
{
    var sPath = window.location.pathname;
    var sPage = sPath.substring(sPath.lastIndexOf('/') + 1);
    if (sPage == "EditForm.aspx")
    return true;
    else return false;
}

And this one will tell you if we are in edit mode on publishing, web part or wiki pages

var isinEditMode = function() {
    var isEditMode = false;
    try {
        var inDesignMode = document.forms[window.MSOWebPartPageFormName].MSOLayout_InDesignMode.value;

        if (inDesignMode == "1") {
            isEditMode = true;

        }
    } catch(e) {
    }

    try {
        var wikiInEditMode = document.forms[window.MSOWebPartPageFormName]._wikiPageMode.value;
        if (wikiInEditMode == "Edit") {
            isEditMode = true;
        }
    } catch(e) {

    }

    return isEditMode;
};

Applying custom page to all subsites

You've create a custom master page and want to apply it to all of your subsites. Use this link to do just that:

/_layouts/changesitemasterpage.aspx

Warning: Publishing Infrastructure Feature must be enabled on your site collection.