resolve-url.html

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@license
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This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
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<link rel="import" href="boot.html">

<script>

  (function() {
    'use strict';

    let CSS_URL_RX = /(url\()([^)]*)(\))/g;
    let ABS_URL = /(^\/)|(^#)|(^[\w-\d]*:)/;
    let DATA_URI = /data:/;
    let workingURL;
    let resolveDoc;
    /**
     * Resolves the given URL against the provided `baseUri'.
     * 
     * Note that this function performs no resolution for URLs that start
     * with `/` (absolute URLs) or `#` (hash identifiers).  For general purpose
     * URL resolution, use `window.URL`.
     *
     * @memberof Polymer.ResolveUrl
     * @param {string} url Input URL to resolve
     * @param {?string=} baseURI Base URI to resolve the URL against
     * @return {string} resolved URL
     */
    function resolveUrl(url, baseURI) {
      if (baseURI && DATA_URI.test(baseURI)) {
        return url;
      }
      if (url && ABS_URL.test(url)) {
        return url;
      }
      // Lazy feature detection.
      if (workingURL === undefined) {
        workingURL = false;
        try {
          const u = new URL('b', 'http://a');
          u.pathname = 'c%20d';
          workingURL = (u.href === 'http://a/c%20d');
        } catch (e) {
          // silently fail
        }
      }
      if (!baseURI) {
        baseURI = document.baseURI || window.location.href;
      }
      if (workingURL) {
        return (new URL(url, baseURI)).href;
      }
      // Fallback to creating an anchor into a disconnected document.
      if (!resolveDoc) {
        resolveDoc = document.implementation.createHTMLDocument('temp');
        resolveDoc.base = resolveDoc.createElement('base');
        resolveDoc.head.appendChild(resolveDoc.base);
        resolveDoc.anchor = resolveDoc.createElement('a');
        resolveDoc.body.appendChild(resolveDoc.anchor);
      }
      resolveDoc.base.href = baseURI;
      resolveDoc.anchor.href = url;
      return resolveDoc.anchor.href || url;

    }

    /**
     * Resolves any relative URL's in the given CSS text against the provided
     * `ownerDocument`'s `baseURI`.
     *
     * @memberof Polymer.ResolveUrl
     * @param {string} cssText CSS text to process
     * @param {string} baseURI Base URI to resolve the URL against
     * @return {string} Processed CSS text with resolved URL's
     */
    function resolveCss(cssText, baseURI) {
      return cssText.replace(CSS_URL_RX, function(m, pre, url, post) {
        return pre + '\'' +
          resolveUrl(url.replace(/["']/g, ''), baseURI) +
          '\'' + post;
      });
    }

    /**
     * Returns a path from a given `url`. The path includes the trailing
     * `/` from the url.
     *
     * @memberof Polymer.ResolveUrl
     * @param {string} url Input URL to transform
     * @return {string} resolved path
     */
    function pathFromUrl(url) {
      return url.substring(0, url.lastIndexOf('/') + 1);
    }

    /**
     * Module with utilities for resolving relative URL's.
     *
     * @namespace
     * @memberof Polymer
     * @summary Module with utilities for resolving relative URL's.
     */
    Polymer.ResolveUrl = {
      resolveCss: resolveCss,
      resolveUrl: resolveUrl,
      pathFromUrl: pathFromUrl
    };

  })();

</script>