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wait_key

Definition: integer i = wait_key()
Description: Return the next key pressed by the user.
Do not return until a key is pressed.
pwa/p2js: Not supported, however since it is very useful at the end of a (desktop) program to stop the console from vanishing, and rather than forcing you to put "if platform()!=JS then" around every call, the (fake) version in p2js.js simply does nothing.
Comments: You could achieve the same result using get_key() as follows: 

    while 1 do
        k = get_key()
        if k!=-1 then
            exit
        end if
    end while

However, this "busy waiting" would tend to slow the system down, whereas wait_key() lets the operating system do other useful work while your program is waiting for the user to press a key.

You could also use getc(0), assuming file number 0 was input from the keyboard, except that would not pick up the special codes for function keys, arrow keys etc.

Note that wait_key() is intended for console applications only and usually has no place in a gui application.
It can only catch keystrokes when the console (assuming you have one) has focus;
when the gui has focus then the gui absorbs any keystrokes, as you should expect.
One obvious exception is that sometimes you might use wait_key() at the very end of a run, after the gui has closed, to prevent the console and any messages on it from immediately dissappearing.

As of 1.0.2 can also be invoked as a procedure, ie a lone wait_key() is treated as {} = wait_key(), no longer needing the explicit discard as it did it prior versions (see pmain.e/Call() and search for T_wait_key).
Example: demo\key.exw
Implementation: via :%opWaitKey in builtins\VM\pfileioN.e (an autoinclude) - be warned however it is low-level complicated stuff that you do not need to know.
See Also: get_key, getc