Using canvas to fix SVG scaling in Internet Explorer

Internet Explorer 9–11 suffer from various bugs that prevent proper scaling of inline SVG’s. This is particularly problematic for SVG icons with variable widths. This is the canvas-based hack I’ve been using to work around the issue.

A popular way to use SVG icons is to generate a spritemap of SVG symbol‘s that you then reference from elsewhere in a document. Most articles on the topic assume your icon dimensions are uniformly square. Twitter’s SVG icons (crafted by @sofo) are variable width, to produce consistent horizontal whitespace around the vectors.

Most browsers will preserve the intrinsic aspect ratio of an SVG. Ideally, I want to set a common height for all the icons (e.g., 1em), and let the browser scale the width of each icon proportionally. This also makes it easy to resize icons in particular contexts – just change the height.

Unfortunately, IE 9–11 do not preserve the intrinsic aspect ratio of an inline SVG. The svg element will default to a width of 300px (the default for replaced content elements). This means it’s not easy to work with variable-width SVG icons. No amount of CSS hacking fixed the problem, so I looked elsewhere – and ended up using canvas.

canvas and aspect ratios

A canvas element – with height and width attributes set – will preserve its aspect ratio when one dimension is scaled. The example below sets a 3:1 aspect ratio.

<canvas height="1" width="3"></canvas>

You can then scale the canvas by changing either dimension in CSS.

canvas {
  display: block;
  height: 2rem;
}

Demo: proportional scaling of canvas.

Fixing SVG scaling in IE

This makes canvas useful for creating aspect ratios. Since IE doesn’t preserve the intrinsic aspect ratio of SVG icons, you can use canvas as a shim. A canvas of the correct aspect ratio provides a scalable frame. The svg can then be positioned to fill the space created by this frame.

The HTML is straightforward:

<div class="Icon" role="img" aria-label="Twitter">
  <canvas class="Icon-canvas" height="1" width="3"></canvas>
  <svg class="Icon-svg">
    <use fill="currentcolor" xlink:href="#icon-twitter"></use>
  </svg>
</div>

So is the CSS:

.Icon {
  display: inline-block;
  height: 1em; /* default icon height */
  position: relative;
  user-select: none;
}

.Icon-canvas {
  display: block;
  height: 100%;
  visibility: hidden;
}

.Icon-svg {
  height: 100%;
  left: 0;
  position: absolute;
  top: 0;
  width: 100%;
}

Setting the canvas height to 100% means it will scale based on the height of the component’s root element – just as SVG’s do in non-IE browsers. Changing the height of the Icon element scales the inner SVG icon while preserving its 3:1 aspect ratio.

Demo: proportional scaling of svg in IE.

Creating an Icon component

The hack is best added to (and eventually removed from) an existing icon component’s implementation.

If you’re generating and inlining an SVG spritemap, you will need to extract the height and width (usually from viewBox) of each of your icons during the build step. If you’re already using the gulp-svgstore plugin, it supports extracting metadata.

Those dimensions need to be set on the canvas element to produce the correct aspect ratio for a given icon.

Example React component (built with webpack):

import iconData from './lib/icons-data.json';
import React from 'react';
import './index.css';

class Icon extends React.Component {
  render() {
    const props = this.props;
    const height = iconData[props.name.height];
    const width = iconData[props.name.width];

    // React doesn't support namespaced attributes, so we have to set the
    // 'use' tag with innerHTML
    const useTag = `<use fill="currentcolor"
                      xlink:href="#icon-${props.name}">
                    </use>`;

    return (
      <span className="Icon">
        <canvas className="Icon-canvas"
          height={height}
          width={width}
        />
        <svg className="Icon-svg"
          dangerouslySetInnerHTML={{__html: useTag}}
          key={props.name}
        />
      </span>
    );
  }
}

export default Icon;

When I introduced this hack to a code base at Twitter, it had no impact on the the rest of the team or the rest of the code base – one of the many benefits of a component-based UI.

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