How to convert a function into a method that can be directly applied to each object.

For example:
I have a function that adjusts the width of selected table to the width of the textFrame.
Now I have selected a textFrame.
I used a loop.
I want each table to apply this once:

tbs[i].tabFitToTextFrame()

If you remove tbs[i] and use:

tabFitToTextFrame();

only the first tab will be modified.

I try Like this:

It doesn't seem to be working.

//var sel = app.documents[0].selection;
var doc = app.activeDocument;
var tbs = doc.selection[0].tables;
//var tbs = getTable(doc.selection);
alert(tbs);
//alert(sel.tables);
if (tbs.length > 0) {
    for (var i = 0; i < tbs.length; i++) {
        alert(tbs[i]);
        tabFitToTextFrame();
    }
}

function tabFitToTextFrame() {
    var doc = app.activeDocument,
        item = doc.selection[0];
    var myTable = getTable(doc.selection);
    parentWidth = myTable.parent.geometricBounds[3] - myTable.parent.geometricBounds[1];
    factor = parentWidth / myTable.width;
    for (col = 0; col < myTable.columns.length; col++)
        myTable.columns[col].width *= factor;
}

function getTable(obj) {
    if (obj == undefined)
        return;
    if ('Array' === obj.constructor.name) {
        for (var i = 0, table; i < obj.length; i++) {
            table = getTable(obj[i]);
            if (table && table.isValid)
                return table;
        }
        return;
    }
    if (obj.constructor.name == 'Cell')
        return obj.parent;
    if (obj.parent.constructor.name == 'Cell')
        return obj.parent.parent;
    if (
        obj.hasOwnProperty('cells')
        && obj.cells.length > 0
    )
        return obj.cells[0].parent;
    if (
        obj.hasOwnProperty('tables')
        && 0 !== obj.tables.length
    )
        return obj.tables[0];
};