Defer execution for ES6 Template Literals

Question

I am playing with the new ES6 Template Literals feature and the first thing that came to my head was a String.format for JavaScript so I went about implementing a prototype:

String.prototype.format = function() {
  var self = this;
  arguments.forEach(function(val,idx) {
    self["p"+idx] = val;
  });
  return this.toString();
};
console.log(`Hello, ${p0}. This is a ${p1}`.format("world", "test"));

ES6Fiddle

However, the Template Literal is evaluated before it's passed to my prototype method. Is there any way I can write the above code to defer the result until after I have dynamically created the elements?

Answer 1

I can see three ways around this:

• Use template strings like they were designed to be used, without any format function:

  console.log(`Hello, ${"world"}. This is a ${"test"}`);
  // might make more sense with variables:
  var p0 = "world", p1 = "test";
  console.log(`Hello, ${p0}. This is a ${p1}`);
  

  or even function parameters for actual deferral of the evaluation:

  const welcome = (p0, p1) => `Hello, ${p0}. This is a ${p1}`;
  console.log(welcome("world", "test"));
  

• Don't use a template string, but a plain string literal:

  String.prototype.format = function() {
      var args = arguments;
      return this.replace(/\$\{p(\d)\}/g, function(match, id) {
          return args[id];
      });
  };
  console.log("Hello, ${p0}. This is a ${p1}".format("world", "test"));
  

• Use a tagged template literal. Notice that the substitutions will still be evaluated without interception by the handler, so you cannot use identifiers like p0 without having a variable named so. This behavior may change if a different substitution body syntax proposal is accepted (Update: it was not).

  function formatter(literals, ...substitutions) {
      return {
          format: function() {
              var out = [];
              for(var i=0, k=0; i < literals.length; i++) {
                  out[k++] = literals[i];
                  out[k++] = arguments[substitutions[i]];
              }
              out[k] = literals[i];
              return out.join("");
          }
      };
  }
  console.log(formatter`Hello, ${0}. This is a ${1}`.format("world", "test"));
  // Notice the number literals: ^               ^