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Jan 7, 2019
Jul 13, 2021

ShowModalDialog Polyfill

This is a window.showModalDialog() shim using a modal HTML5 <dialog> element and ECMAScript 2015 Generators or ECMAScript 2017 Async/Await. It was tested in the latest Google Chrome and in the latest Mozilla Firefox with the dom.dialog_element.enabled preference set to true in about:config. Just include the following HTML code before using showModalDialog function:

<script src=""></script>

Passing both window.dialogArguments and window.returnValue is supported, provided that the dialog document is on the same server as the host document.

ShowModalDialog Polyfill is using Promises, Generators, yield, async, await and the spawn function by Jake Archibald. If they are unavailable, the polyfill is using eval and JSON as a fallback, provided that statements are separated by new lines, the showModalDialog function is not nested and runs only once in a function.

Please note that native showModalDialog support is considered deprecated and it was removed from Chrome 43 and Firefox 56.


spawn(function*() {

    //statements before showing a modal dialog

    var returnValue = yield window.showModalDialog( url [, arguments, options] );

    //statements after closing a modal dialog



(async function() {

    //statements before showing a modal dialog

    var returnValue = await window.showModalDialog( url [, arguments, options] );

    //statements after closing a modal dialog



(function() {

    //statements before showing a modal dialog

    var returnValue = window.showModalDialog( url [, arguments, options] );

    //statements after closing a modal dialog



  • url - a string that specifies the URL of the document to load and display;
  • arguments - a variant that specifies the arguments to use when displaying the document;
  • options - a string that specifies the dialog box style, using CSS or the following semicolon-delimited attributes: dialogHeight:???px;dialogLeft:???px;dialogTop:???px;dialogWidth:???px;

When using generators or async/await, both showModalDialog and spawn functions are Promises, so you can use their then method and yield them.

When using an eval fallback, the showModalDialog function throws an exception to stop executing code until the modal is closed, then it evals the remaining code of a caller function.

In order to close the dialog from inside of it, invoke parent.document.getElementsByTagName('dialog')[0].close(); provided that both documents have the same origin.


Here is a live demo. Works best in Google Chrome.


ShowModalDialog Polyfill is developed by Jerzy Głowacki under Apache 2.0 License.


window.showModalDialog polyfill using a <dialog> element







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