Python-like template string in ECMAScript 6

es6-tagged-template.js

// Python-like template string in ECMAScript 6 "tagged template"
// inspired from http://stackoverflow.com/a/22619256
function formatter(literals, ...substitutions) {
  return {
    format: function() {
      let out = [], i = 0, k = 0;
      for (i,k; i < literals.length; i++) {
        out[k++] = literals[i];
        out[k++] = Number.isInteger(substitutions[i]) ?
          arguments[substitutions[i]] :
          arguments[0][substitutions[i]];
      }
      out[k] = literals[i];
      return out.join("");
    },
  };
};
console.log(formatter`Hello, ${0}. This is a ${1}`.format("world", "test"));
console.log(formatter`Hello, ${"foo"}. This is a ${"bar"}`.format({foo: "world", bar: "test"}));