The Window.resizeBy() method resizes the current window by a specified amount.

• xDelta is the number of pixels to grow the window horizontally.
• yDelta is the number of pixels to grow the window vertically.

None (undefined).

This method resizes the window relative to its current size. To resize the window in absolute terms, use window.resizeTo().

For security reasons, it's no longer possible in Firefox for a website to change the default size of a window in a browser if the window wasn't created by window.open(), or contains more than one tab. See the compatibility table for details on the change.

Even if you create window by window.open() it is not resizable by default. To make the window resizable, you must open it with the "resizable" feature.

// Create resizable window
myExternalWindow = window.open(
  "https://example.com",
  "myWindowName",
  "resizable",
);

// Resize window to 500x500
myExternalWindow.resizeTo(500, 500);

// Make window relatively smaller to 400x400
myExternalWindow.resizeBy(-100, -100);

The window you create must respect the Same Origin Policy. If the window you open is not in the same origin as the current window, you will not be able to resize, or access any information on, that window/tab.