Changing the keypress

Here a simple example to replace , to . using Vanilla JavaScript

// change ',' to '.'
document.getElementById('only_float').addEventListener('keypress', function(e){
    if (e.key === ','){
        // get old value
        var start = e.target.selectionStart;
        var end = e.target.selectionEnd;
        var oldValue = e.target.value;

        // replace point and change input value
        var newValue = oldValue.slice(0, start) + '.' + oldValue.slice(end)
        e.target.value = newValue;

        // replace cursor
        e.target.selectionStart = e.target.selectionEnd = start + 1;

        e.preventDefault();
    }
})

<input type="text" id="only_float" />

You can't change the character or key associated with a key event, or fully simulate a key event. However, you can handle the keypress yourself and manually insert the character you want at the current insertion point/caret. I've provided code to do this in a number of places on Stack Overflow. For a contenteditable element:

• Need to set cursor position to the end of a contentEditable div, issue with selection and range objects

Here's a jsFiddle example: http://www.jsfiddle.net/Ukkmu/4/

For an input:

• Can I conditionally change the character entered into an input on keypress?

• show different keyboard character from the typed one in google chrome

Cross-browser jsFiddle example: http://www.jsfiddle.net/EXH2k/6/

IE >= 9 and non-IE jsFiddle example: http://www.jsfiddle.net/EXH2k/7/

My solution example (change in input[type=text] the character ',' to '.'):

element.addEventListener('keydown', function (event) {
if(event.key === ','){
  setTimeout(function() {
    event.target.value += '.';
  }, 4);
  event.preventDefault();
};