How to convert a char to its keycode?

What do you mean by "keyCode"? Different browsers have different keyCode values in keyup and keydown events which will not necessarily correspond to the ASCII code for the corresponding character. For alphanumeric keys, the keypress event will give you the ASCII code in most browsers via the charCode or which properties. This page is useful.

Update September 2015

As pointed out by Jan in the comments, keyCode will eventually be superseded by the superior key property. However, there is not much browser support for this yet.

You can use the charCodeAt function to achieve this.

Working example:

function showKeyCode () {
    var character = document.getElementById("character").value.substring(0, 1);
    var code = document.getElementById("character").value.charCodeAt(0);
    var msg = "The Key Code for the \"" + character + "\" character is " + code + ".";
    alert(msg);
}

<input type="text" id="character" size="15">
<input type="button" value="Show Key Code" onclick="showKeyCode();">

Here is a object with some keys and their corresponding keycodes:

{"0":48,"1":49,"2":50,"3":51,"4":52,"5":53,"6":54,"7":55,"8":56,"9":57,"d":68,"b":66,"a":65,"s":83,"i":73,"f":70,"k":75,"ß":219,"Dead":220,"+":187,"ü":186,"p":80,"o":79,"u":85,"z":90,"t":84,"r":82,"e":69,"w":87,"g":71,"h":72,"j":74,"l":76,"ö":192,"ä":222,"#":191,"y":89,"x":88,"c":67,"v":86,"n":78,"m":77,",":188,".":190,"-":189,"ArrowRight":39,"ArrowLeft":37,"ArrowUp":38,"ArrowDown":40,"PageDown":34,"Clear":12,"Home":36,"PageUp":33,"End":35,"Delete":46,"Insert":45,"Control":17,"AltGraph":18,"Meta":92,"Alt":18,"Shift":16,"CapsLock":20,"Tab":9,"Escape":27,"F1":112,"F2":113,";":188,":":190,"_":189,"'":191,"*":187,"Q":81,"W":87,"E":69,"R":82,"T":84,"Z":90,"S":83,"A":65,"D":68,"I":73,"U":85,"O":79,"Y":89,"X":88,"C":67,"F":70,"V":86,"G":71,"B":66,"H":72,"N":78,"J":74,"M":77,"K":75,"L":76,"P":80,"Ö":192,"Ä":222,"Ü":186,"!":49,"\"":50,"§":51,"$":52,"%":53,"&":54,"/":55,"(":56,")":57,"=":48,"?":219,"°":220}

if some keys are missing or are different for your browser, you can use the following function:

let a = {}
document.addEventListener("keydown", ({key, keyCode}) => a[key] = keyCode);

Just copy it to the console and smash your head on the keyboard. The result will be stored in a