url filter should detect absolute urls without the protocol

[oscarotero]

Is your feature request related to a problem? Please describe.
I'm using the url filter for images in this way:

<img src="{{ image.src | url }}">

But sometimes, the images are external urls. To make them work in http and https contexts, I remove the protocol, so the urls are like this: //placehold.it/300x100. Seems like the filter does not detect this and generate the invalid url /placehold.it/300x100.

Describe the solution you'd like
I can see that the filter can detect http and https protocols (https://github.com/11ty/eleventy/blob/master/src/Filters/Url.js#L8) so I propose adding another condition: url.indexOf("//") === 0.

[denisbrodbeck]

You could write a wrapper filter and use that instead.

// .eleventy.js

const url = require(`@11ty/eleventy/src/Filters/Url`);

module.exports = function (config) {
  config.addFilter(`myurl`, (uri) => {
    if( condition) {
        return myspecialcase;
    }
    return url(uri);
  });

  // Base Config
  return {
    dir: {
      input: `src`,
      output: `dist`,
      includes: `_includes`,
      layouts: `_layouts`,
      data: `_data`,
    },
    templateFormats: [`njk`, `md`, `11ty.js`],
    htmlTemplateEngine: `njk`,
    markdownTemplateEngine: `njk`,
  };
};

Then use it instead of url:

<img src="{{ image.src | myurl }}">

[oscarotero]

@denisbrodbeck Yep, this would be a workaround, but I think this is a malfunction of the filter, so I'd rather fix it.

[denisbrodbeck]

Sure, fire away with a PR 😄

[zachleat]

Thanks for the PR! It’s merged and will ship with 1.0