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I'm trying to change the value of the top margin when hitting the up arrow key the code seems right to me but i don't know why it wont work !

JavaScript 

var playerPosition = 0;

window.onkeyup = function(e) {
    var key = e.keyCode ? e.keyCode : e.which;

    if(key = 38) {
        playerPosition += 10;

    } else if(key = 40) {
        playerPosition -= 10;
    }

    document.getElementsByClassName('player').style.marginTop = playerPosition+".px";

 }

html/CSS

.mainDiv {
        display: block;
        position: absolute;
        margin: auto;
        top: 0;
        bottom: 0;
        left: 0;
        right: 0;
        background-color: black;
        width: 600px;
        height: 400px;
    }
    .player {
        position: absolute;
        background-color: #FFF;
        width: 5px;
        height: 70px;
        margin: 20px 0 0 10px;
        padding: 0 0 0 0;
    }
    .bar {
        top: 30px;
        height: 100%;
        width: 5px;
        border-style: dashed;
        border-left: 5px;
        border-color: #FFF;
        position: fixed;
        left: 50%;
    }
    .ai {
        position: absolute;
        right: 10px;
        background-color: #FFF;
        width: 5px;
        height: 70px;
        margin: 164px 0 0 10px;
        padding: 0 0 0 0;
    }

    .ball {
        position: absolute;
        left: 50px;
        bottom: 69px;
        width: 20px;
        height: 20px;
        border-radius: 50%;
        background-color: #FFF;
    }
    </style>
    <script src="sprite.js" defer="defer"></script>
</head>
<body>
    <div class="mainDiv">
        <div class="player"></div>
        <div class="bar"></div>
        <div class="ai"></div>
        <div class="ball"></div>
    </div>
</body>
</html>

I'm trying to change the value of the top margin when hitting the up arrow key the code seems right to me but i don't know why it wont work !

JavaScript 

var playerPosition = 0;

window.onkeyup = function(e) {
    var key = e.keyCode ? e.keyCode : e.which;

    if(key = 38) {
        playerPosition += 10;

    } else if(key = 40) {
        playerPosition -= 10;
    }

    document.getElementsByClassName('player').style.marginTop = playerPosition+".px";

 }

html/CSS

.mainDiv {
        display: block;
        position: absolute;
        margin: auto;
        top: 0;
        bottom: 0;
        left: 0;
        right: 0;
        background-color: black;
        width: 600px;
        height: 400px;
    }
    .player {
        position: absolute;
        background-color: #FFF;
        width: 5px;
        height: 70px;
        margin: 20px 0 0 10px;
        padding: 0 0 0 0;
    }
    .bar {
        top: 30px;
        height: 100%;
        width: 5px;
        border-style: dashed;
        border-left: 5px;
        border-color: #FFF;
        position: fixed;
        left: 50%;
    }
    .ai {
        position: absolute;
        right: 10px;
        background-color: #FFF;
        width: 5px;
        height: 70px;
        margin: 164px 0 0 10px;
        padding: 0 0 0 0;
    }

    .ball {
        position: absolute;
        left: 50px;
        bottom: 69px;
        width: 20px;
        height: 20px;
        border-radius: 50%;
        background-color: #FFF;
    }
    </style>
    <script src="sprite.js" defer="defer"></script>
</head>
<body>
    <div class="mainDiv">
        <div class="player"></div>
        <div class="bar"></div>
        <div class="ai"></div>
        <div class="ball"></div>
    </div>
</body>
</html>
  • javascript
  • html
  • css
Share Improve this question asked Dec 27, 2014 at 2:41 ZEEZEE 5,8794 gold badges39 silver badges54 bronze badges 2
  • possible duplicate of GetElementsByClassName Not Working As Expected – user663031 Commented Dec 27, 2014 at 7:52
  • See also stackoverflow./questions/24292561/…, stackoverflow./questions/19289907/…, stackoverflow./questions/10693845/…, etc. – user663031 Commented Dec 27, 2014 at 7:54
Add a ment  | 

4 Answers 4

Reset to default 2

document.getElementsByClassName('player') returns a NodeList of elements (array-like) that have the class player. You need to loop through the list and apply the style changes to each:

var players = document.getElementsByClassName('player');
for(var i = 0; i < players.length; i++)
    players[i].style.marginTop = playerPosition+"px";

Or I guess if you only have one, apply it to the 0th element.

fiddle

Function is called "getElements[...]", plural, so it returns an array of HTML elements with class name provided. Moreover in your conditional statements you used assignment operator (=) instead of parison operator (==). jsfiddle

document.getElementsByClassName('player')[0].style.marginTop = playerPosition+"px";

getElementsByClassName returns a HTMLCollection. So, you have to specify the element to work.

for example:

document.getElementsByClassName('player')[0].style.marginTop = playerPosition+".px";

document.getElementById("MyElement").classList.add('MyClass');
document.getElementById("MyElement").classList.remove('MyClass');
if ( document.getElementById("MyElement").classList.contains('MyClass') )
    document.getElementById("MyElement").classList.toggle('MyClass');

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