Template literal tag function generator

templateLiteralTagGenerator.js

/**
 * @file generateTemplateLiteralTag gist.
 * @author Ian Sanders <[email protected]>
 * @license
 * Public Domain - Attribution is nice but not at all required. Feel free to do
 * anything with this code.
 */

/**
 * Generate a template literal tag that simply applies the given function on
 * each placeholder value in the given template, using the placeholder value as
 * the first argument and any other given arguments after that.
 * @param {function(*, ...*): string} functionToApply The function that would
 * be applied to the template placeholders. Should take a placeholder as the first
 * argument and any number of arguments after that, and should always return
 * a string or value that can be easily and predictably converted to a string.
 * @param {...*} [extraArgs] Optional extra ordered arguments to pass to `functionToApply`.
 * @returns {function(string[], ...*): string} A function that can be used 
 * as a tag for a string template literal.
 * @example <caption>URL Parameter Escaper</caption>
 * let urlEscape = generateTemplateLiteralTag(encodeURIComponent);
 * let queryString = "This=will &=be ?=safely escaped!";
 * console.log(urlEscape`http://example.com?q=${queryString}&p=1`);
 * // http://example.com?q=This%3Dwill%20%26%3Dbe%20%3F%3Dsafely%20escaped!&p=1
 * @example <caption>Lowercaser</caption>
 * function lcString(string) { return string.toLowerCase(); }
 * let lcPlaceholders = generateTemplateLiteralTag(lcString);
 * let strings = ["StRiNg A", "STRING B", "String C"];
 * console.log(lcPlaceholders`In lowercase, your strings are ${strings.join(", ")}.`);
 * // In lowercase, your strings are string a, string b, string c.
 */
function generateTemplateLiteralTag(functionToApply, ...extraArgs) {
  return function(strings, ...placeholders) {
    return strings.reduce((result, string, i) => {
      return (result + functionToApply(placeholders[i - 1], ...extraArgs) + string);
    });
  };
}