Disallow non-templates as interpolations in Polymer.html

lib/utils/html-tag.html

@@ -12,28 +12,68 @@
   (function() {
     'use strict';
 
+    /**
+     * Class representing a static string value which can be used to filter
+     * strings by asseting that they have been created via this class. The
+     * `value` property returns the string passed to the constructor.
+     */
+    class LiteralString {
+      constructor(string) {
+        /** @type {string} */
+        this.value = string.toString();
+      }
+      /**
+       * @return {string} LiteralString string value
+       */
+      toString() {
+        return this.value;
+      }
+    }
+
+    /**
+     * @param {*} value Object to stringify into HTML
+     * @return {string} HTML stringified form of `obj`
+     */
+    function literalValue(value) {
+      if (value instanceof LiteralString) {
+        return /** @type {!LiteralString} */(value).value;
+      } else {
+        throw new Error(`non-literal value passed to Polymer.htmlLiteral: ${value}`);
+      }
+    }
+
     /**
      * @param {*} value Object to stringify into HTML
      * @return {string} HTML stringified form of `obj`
      */
     function htmlValue(value) {
       if (value instanceof HTMLTemplateElement) {
-        return /** @type {!HTMLTemplateElement} */(value).innerHTML;
+        return /** @type {!HTMLTemplateElement } */(value).innerHTML;
+      } else if (value instanceof LiteralString) {
+        return literalValue(value);
       } else {
-        return String(value);
+        throw new Error(`non-template value passed to Polymer.html: ${value}`);
       }
     }
 
     /**
-     * A template literal tag that creates an HTML <template> element from the contents of the string.
+     * A template literal tag that creates an HTML <template> element from the
+     * contents of the string.
      *
      * This allows you to write a Polymer Template in JavaScript.
      *
-     * Interpolated values are converted to strings when the template is created,
-     * they are not intended as a replacement for Polymer data-binding.
+     * Templates can be composed by interpolating `HTMLTemplateElement`s in
+     * expressions in the JavaScript template literal. The nested template's
+     * `innerHTML` is included in the containing template.  The only other
+     * values allowed in expressions are those returned from `Polymer.htmlLiteral`
+     * which ensures only literal values from JS source ever reach the HTML, to
+     * guard against XSS risks.
      *
-     * There is special support for HTMLTemplateElement values,
-     * allowing for easy composition of superclass or partial templates.
+     * All other values are disallowed in expressions to help prevent XSS
+     * attacks; however, `Polymer.htmlLiteral` can be used to compose static
+     * string values into templates. This is useful to compose strings into
+     * places that do not accept html, like the css text of a `style`
+     * element.
      *
      * Example:
      *
@@ -54,8 +94,36 @@
     Polymer.html = function html(strings, ...values) {
       const template = /** @type {!HTMLTemplateElement} */(document.createElement('template'));
       template.innerHTML = values.reduce((acc, v, idx) =>
-        acc + htmlValue(v) + strings[idx + 1], strings[0]);
+          acc + htmlValue(v) + strings[idx + 1], strings[0]);
       return template;
     };
+
+    /**
+     * An html literal tag that can be used with `Polymer.html` to compose.
+     * a literal string.
+     *
+     * Example:
+     *
+     *   static get template() {
+     *     return Polymer.html`
+     *       <style>
+     *         :host { display: block; }
+     *         ${styleTemplate}
+     *       </style>
+     *       <div class="shadowed">${staticValue}</div>
+     *       ${super.template}
+     *     `;
+     *   }
+     *   static get styleTemplate() { return Polymer.htmlLiteral`.shadowed { background: gray; }`; }
+     *
+     * @memberof Polymer
+     * @param {!ITemplateArray} strings Constant parts of tagged template literal
+     * @param {...*} values Variable parts of tagged template literal
+     * @return {!LiteralString} Constructed literal string
+     */
+    Polymer.htmlLiteral = function(strings, ...values) {
+      return new LiteralString(values.reduce((acc, v, idx) =>
+          acc + literalValue(v) + strings[idx + 1], strings[0]));
+    };
   })();
 </script>

test/unit/html-tag.html

@@ -32,19 +32,26 @@
         let t = Polymer.html`<span>the \`price\` is \${{foo}}\n</span>`;
         assert.equal(t.innerHTML, '<span>the `price` is ${{foo}}\n</span>');
       });
-      test('variables work', function() {
-        let a = 3;
-        let b = 'foo';
-        let t = Polymer.html`<div>${a} ${b}</div>`;
-        let inst = document.createElement('div');
-        inst.appendChild(t.content.cloneNode(true));
-        assert.equal(inst.textContent, `${a} ${b}`);
+      test('interpolation of non-templates throw', function() {
+        assert.throws(() => Polymer.html`<div>${'a'}</div>`);
       });
-      test('template variables only include the template contents', function() {
+      test('interpolation of templates include the template contents', function() {
         let t1 = Polymer.html`<div></div>`;
         let t2 = Polymer.html`<span>${t1}</span>`;
         assert.equal(t2.innerHTML, '<span><div></div></span>');
       });
+      test('interpolation of literal values', function() {
+        let s1 = Polymer.htmlLiteral`hello`;
+        assert.equal(s1.value, `hello`);
+        let s2 = Polymer.htmlLiteral`hello${Polymer.htmlLiteral` ${Polymer.htmlLiteral`world`}!`}!`;
+        assert.equal(s2.value, 'hello world!!');
+      });
+      test('interpolation of literal values within html values', function() {
+        let s1 = Polymer.htmlLiteral`foo`;
+        let s2 = Polymer.htmlLiteral`display: block;`;
+        let t = Polymer.html`<style>:host {${s2}}</style><div ${s1}></div>`;
+        assert.equal(t.innerHTML, `<style>:host {display: block;}</style><div foo=""></div>`);
+      });
     });
     suite('Polymer + html fn', function() {
       suiteSetup(function() {
@@ -131,4 +138,4 @@
     });
   </script>
 </body>
-</html>
+</html>

types/lib/utils/html-tag.d.ts

@@ -10,19 +10,41 @@
 
 /// <reference path="boot.d.ts" />
 
+/**
+ * Class representing a static string value which can be used to filter
+ * strings by asseting that they have been created via this class. The
+ * `value` property returns the string passed to the constructor.
+ */
+declare class LiteralString {
+  value: string;
+
+  /**
+   * @returns LiteralString string value
+   */
+  toString(): string;
+}
+
 declare namespace Polymer {
 
 
   /**
-   * A template literal tag that creates an HTML <template> element from the contents of the string.
+   * A template literal tag that creates an HTML <template> element from the
+   * contents of the string.
    *
    * This allows you to write a Polymer Template in JavaScript.
    *
-   * Interpolated values are converted to strings when the template is created,
-   * they are not intended as a replacement for Polymer data-binding.
+   * Templates can be composed by interpolating `HTMLTemplateElement`s in
+   * expressions in the JavaScript template literal. The nested template's
+   * `innerHTML` is included in the containing template.  The only other
+   * values allowed in expressions are those returned from `Polymer.htmlLiteral`
+   * which ensures only literal values from JS source ever reach the HTML, to
+   * guard against XSS risks.
    *
-   * There is special support for HTMLTemplateElement values,
-   * allowing for easy composition of superclass or partial templates.
+   * All other values are disallowed in expressions to help prevent XSS
+   * attacks; however, `Polymer.htmlLiteral` can be used to compose static
+   * string values into templates. This is useful to compose strings into
+   * places that do not accept html, like the css text of a `style`
+   * element.
    *
    * Example:
    *
@@ -38,4 +60,27 @@ declare namespace Polymer {
    * @returns Constructed HTMLTemplateElement
    */
   function html(strings: TemplateStringsArray, ...values: any[]): HTMLTemplateElement;
+
+
+  /**
+   * An html literal tag that can be used with `Polymer.html` to compose.
+   * a literal string.
+   *
+   * Example:
+   *
+   *   static get template() {
+   *     return Polymer.html`
+   *       <style>
+   *         :host { display: block; }
+   *         ${styleTemplate}
+   *       </style>
+   *       <div class="shadowed">${staticValue}</div>
+   *       ${super.template}
+   *     `;
+   *   }
+   *   static get styleTemplate() { return Polymer.htmlLiteral`.shadowed { background: gray; }`; }
+   *
+   * @returns Constructed literal string
+   */
+  function htmlLiteral(strings: TemplateStringsArray, ...values: any[]): LiteralString;
 }