[javascript] How can you find the height of text on an HTML canvas?

The canvas spec doesn't give us a method for measuring the height of a string. However, you can set the size of your text in pixels and you can usually figure out what the vertical bounds are relatively easily.

If you need something more precise then you could throw text onto the canvas and then get pixel data and figure out how many pixels are used vertically. This would be relatively simple, but not very efficient. You could do something like this (it works, but draws some text onto your canvas that you would want to remove):

function measureTextHeight(ctx, left, top, width, height) {

    // Draw the text in the specified area
    ctx.save();
    ctx.translate(left, top + Math.round(height * 0.8));
    ctx.mozDrawText('gM'); // This seems like tall text...  Doesn't it?
    ctx.restore();

    // Get the pixel data from the canvas
    var data = ctx.getImageData(left, top, width, height).data,
        first = false, 
        last = false,
        r = height,
        c = 0;

    // Find the last line with a non-white pixel
    while(!last && r) {
        r--;
        for(c = 0; c < width; c++) {
            if(data[r * width * 4 + c * 4 + 3]) {
                last = r;
                break;
            }
        }
    }

    // Find the first line with a non-white pixel
    while(r) {
        r--;
        for(c = 0; c < width; c++) {
            if(data[r * width * 4 + c * 4 + 3]) {
                first = r;
                break;
            }
        }

        // If we've got it then return the height
        if(first != r) return last - first;
    }

    // We screwed something up...  What do you expect from free code?
    return 0;
}

// Set the font
context.mozTextStyle = '32px Arial';

// Specify a context and a rect that is safe to draw in when calling measureTextHeight
var height = measureTextHeight(context, 0, 0, 50, 50);
console.log(height);

For Bespin they do fake a height by measuring the width of a lowercase 'm'... I don't know how this is used, and I would not recommend this method. Here is the relevant Bespin method:

var fixCanvas = function(ctx) {
    // upgrade Firefox 3.0.x text rendering to HTML 5 standard
    if (!ctx.fillText && ctx.mozDrawText) {
        ctx.fillText = function(textToDraw, x, y, maxWidth) {
            ctx.translate(x, y);
            ctx.mozTextStyle = ctx.font;
            ctx.mozDrawText(textToDraw);
            ctx.translate(-x, -y);
        }
    }

    if (!ctx.measureText && ctx.mozMeasureText) {
        ctx.measureText = function(text) {
            ctx.mozTextStyle = ctx.font;
            var width = ctx.mozMeasureText(text);
            return { width: width };
        }
    }

    if (ctx.measureText && !ctx.html5MeasureText) {
        ctx.html5MeasureText = ctx.measureText;
        ctx.measureText = function(text) {
            var textMetrics = ctx.html5MeasureText(text);

            // fake it 'til you make it
            textMetrics.ascent = ctx.html5MeasureText("m").width;

            return textMetrics;
        }
    }

    // for other browsers
    if (!ctx.fillText) {
        ctx.fillText = function() {}
    }

    if (!ctx.measureText) {
        ctx.measureText = function() { return 10; }
    }
};