Determine Pixel Length of String in Javascript/jQuery?

Wrap text in a span and use jquery width()

The contexts used for HTML Canvases have a built-in method for checking the size of a font. This method returns a TextMetrics object, which has a width property that contains the width of the text.

function getWidthOfText(txt, fontname, fontsize){
    if(getWidthOfText.c === undefined){
        getWidthOfText.c=document.createElement('canvas');
        getWidthOfText.ctx=getWidthOfText.c.getContext('2d');
    }
    var fontspec = fontsize + ' ' + fontname;
    if(getWidthOfText.ctx.font !== fontspec)
        getWidthOfText.ctx.font = fontspec;
    return getWidthOfText.ctx.measureText(txt).width;
}

Or, as some of the other users have suggested, you can wrap it in a span element:

function getWidthOfText(txt, fontname, fontsize){
    if(getWidthOfText.e === undefined){
        getWidthOfText.e = document.createElement('span');
        getWidthOfText.e.style.display = "none";
        document.body.appendChild(getWidthOfText.e);
    }
    if(getWidthOfText.e.style.fontSize !== fontsize)
        getWidthOfText.e.style.fontSize = fontsize;
    if(getWidthOfText.e.style.fontFamily !== fontname)
        getWidthOfText.e.style.fontFamily = fontname;
    getWidthOfText.e.innerText = txt;
    return getWidthOfText.e.offsetWidth;
}

EDIT 2020: added font name+size caching at Igor Okorokov's suggestion.

I don't believe you can do just a string, but if you put the string inside of a <span> with the correct attributes (size, font-weight, etc); you should then be able to use jQuery to get the width of the span.

<span id='string_span' style='font-weight: bold; font-size: 12'>Here is my string</span>
<script>
  $('#string_span').width();
</script>