Pressing the first button...

results in a different page being loaded in the same window.

That page shows how you can use JavaScript to go back to the previous document,...

how to retrieve the query string ('par=59')...

and how to determine its URL.

The second button,...

when pressed,

results in another document being shown in another window. Could be a new window, could be an existing one or a frame, depending on whether 'windowname' already exists.

The third argument to 'window.open' is a string with options.

/xxx/ and /yyy/ are regular expressions, that's why they're not in quotes. The effect is, that the window's size will be one quarter of the screen resolution.

To close the window using JavaScript.

Refreshing a page can be done in this manner. 'true', because we want a genuine reload, a reload from the server, not from the browser cache.

Clicking the 4th button...

show a couple interesting 'navigator' members in the textarea.

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At times one wishes to avoid cluttering the history. At such a time one can resort to using
window.location.replace('somepage.html');