Question

Both main window and iframe are on the same domain, but what I want to achieve is to resize iframe to fit its contents using js only within that iframe.

I also do not know the id of the iframe that is assigned to it by main window. And I do not want to use jquery or any other framework for it.

Answer 1

You can do this also without knowing the idof the iframe in the parent window:

window.frameElement.style.width = iframeContentWidth + 'px';
window.frameElement.style.height = iframeContentHeight + 'px';

See frameElement at MDN.

EDIT

Just in case the iframe happens to be in a container element which has fixed size and overflow: hidden you can do something like this:

function resizeIframe (iframeContentWidth, iframeContentHeight) {
    var container = window.frameElement.parentElement;
    if (container != parent.document.body) {
        container.style.width = iframeContentWidth + 'px';
        container.style.height = iframeContentHeight + 'px';
    }
    window.frameElement.style.width = iframeContentWidth + 'px';
    window.frameElement.style.height = iframeContentHeight + 'px';
    return;
}