How to alternate Range moveStart in Firefox?

QUESTION

Does anybody know how to use range.setStart in the same way as range.moveStart works in IE? I'd like to implement backspace/delete in JS, something like this:

range.moveStart('character',-1); range.deleteContents();

but in Firefox

ANSWER

Firefox, along with all modern browsers except IE <= 8 uses DOM Ranges. There's no direct analogue to the moveStart method of IE's TextRange and it's tricky to do in the general case. If the range is within a text node and not at the start, it's easy; otherwise you'll need to walk backwards in the document to find the preceding text node and move the range into it. The following only works within a single text node:

function backspace(){
    var sel = window.getSelection();

    // If there is a selection rather than a caret, just delete the selection
    if(!sel.isCollapsed){
        sel.deleteFromDocument();
    }elseif(sel.rangeCount){
        var range = sel.getRangeAt(0);
        if(range.startContainer.nodeType ==3&& range.startOffset >0){
            range.setStart(range.startContainer, range.startOffset -1);
            sel.removeAllRanges();
            sel.addRange(range);
            sel.deleteFromDocument();
        }
    }
}

WebKit and Firefox 4 have the modify method of Selection objects which solves the problem completely:

function backspace2(){
    var sel = window.getSelection();

    // If there is a selection rather than a caret, just delete the selection
    if(!sel.isCollapsed){
        sel.deleteFromDocument();
    }elseif(sel.rangeCount && sel.modify){
        sel.modify("extend","backward","character");
        sel.deleteFromDocument();
    }
}