SharePoint: Use _spBodyOnLoadFunctionNames

tl;dr – In SharePoint, use _spBodyOnLoadFunctionNames instead of $(document).ready

When using JavaScript

How you do it in jQuery

$(document).ready(function() {

// put your code here...

});

How you do it in SharePoint

function loadMyPage() {

// put your code here...

}

_spBodyOnLoadFunctionNames.push("loadMyPage");