pageeditmode.js

_spBodyOnLoadFunctionNames.push("myScriptStartup");
function myScriptStartup () {
     var inDesignMode = document.forms[MSOWebPartPageFormName].MSOLayout_InDesignMode.value;

if (inDesignMode == "1") {
    // page is in edit mode
    // Do things differently here
    console.log('Page is in edit mode. so we will not execute the custom script');
}
else {
        // regular script execution goes here
}

}