javascript - Can someone explain how to use an event listener in the most simple of terms? -

i beginner coder in high school programming class, trying edit html/javascript game found online , update function called acceleration:

function accelerate(n) {     mygamepiece.gravity = n; } 

with keypress or keydown/up event. make box accelerate or down whether spacebar pressed. issue event listener examples can find use buttons, , game works using acceleration button:

 <button onclick="" onkeydown="accelerate(-.5)" onkeyup="accelerate(.5)">start game</button> 

the issue want window / document (i don't know is) detect if user inputting spacebar key , update acceleration function accordingly. appreciate willing give, thank :d

you can (and should) use addeventlistener. it's better way of setting event handlers since isn't mixed in html, can done on dynamic elements , allows multiple handlers single event.

document.addeventlistener('keydown', function(e) {    // prevent space causing page scroll    e.preventdefault();        // nice tool finding key codes: http://keycode.info/    var ispressingspace = (e.keycode === 32);    var ispressinga = (e.keycode === 65); // can use a, if want    if (ispressingspace || ispressinga) {      document.queryselector('span').innertext = 'accelerating';    }  });    document.addeventlistener('keyup', function() {    document.queryselector('span').innertext = 'decelerating';  });

<span>decelerating</span>