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When you want to handle a window, you must make sure it still exists. Before you declare a variable for a window.open() method, declare it as a global variable and set it to null. Remember that the open() method returns the window object of the new window. Here's an example:

var win = null;
function launchWindow() {
  win = window.open();
// statements that refer to the new window go here
}

If you want to perform an operation on the new window, you should first check if the variable win isn't null:

// if win exists, move the window
if (win) win.moveTo(0, 0);

Note that null evaluates to false, while any other valid object evaluates to true. If win evaluates to true, you know for sure that it isn't null, meaning that a new window was successfully launched.

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