Suppose such a snippet of html
<a name="edit" href="javascript:" onclick="edit_column(this, {{ column.id }})"><span class="glyphicon glyphicon-pencil"></span></a>
the js:
function edit_column(the, column_id){
alert(the)}
On clicking, it alerts "javascript:"
I tried to retrieve more info about it with
function edit_column(the, column_id){
alert(typeof the)}
It alert object
How could I retrieve properties and method of such a specified object, like dir or vars in python?
You can use Object.keys to get an array of the object's enumerable own property names (and then use those to look up the values) or, in modern environments, Object.entries which gives you the names and values (as an array of arrays where each of the subordinate arrays is in the form [name, value]). If you need information about non-enumerable properties, there are also Object.getOwnPropertyNames and (if you need to look at its prototype) Object.getPrototypeOf.
But your specific example is a DOM element (an HTMLAnchorElement instance, to be precise), so you can also look up its properties and methods in the spec.
On clicking, it alerts "javascript:"
If you're wondering why that is, it's because alert coerces its argument to string, and the string an HTMLAnchorElement produces when you do that is its href property's value.
alert(Object.keys(the)) , alert(Object.entries(the)) and alert(Object.entries($(the))), it prompt blank with no information.