Lazy Templating

[CMCDragonkai]

Is your feature request related to a problem? Please describe.

Sometimes while logging, you want to use logger.debug, but you end up formatting a message even if the message won't be logged. Would be nice to make the formatting of the message content itself lazy.

Describe the solution you'd like

Use something like:

function template(strings, ...keys) {
  return (...values) => {
    const dict = values[values.length - 1] || {};
    const result = [strings[0]];
    keys.forEach((key, i) => {
      const value = Number.isInteger(key) ? values[key] : dict[key];
      result.push(value, strings[i + 1]);
    });
    return result.join("");
  };
}

const t1Closure = template`${0}${1}${0}!`;
// const t1Closure = template(["","","","!"],0,1,0);
t1Closure("Y", "A"); // "YAY!"

const t2Closure = template`${0} ${"foo"}!`;
// const t2Closure = template([""," ","!"],0,"foo");
t2Closure("Hello", { foo: "World" }); // "Hello World!"

const t3Closure = template`I'm ${"name"}. I'm almost ${"age"} years old.`;
// const t3Closure = template(["I'm ", ". I'm almost ", " years old."], "name", "age");
t3Closure("foo", { name: "MDN", age: 30 }); // "I'm MDN. I'm almost 30 years old."
t3Closure({ name: "MDN", age: 30 }); // "I'm MDN. I'm almost 30 years old."

Describe alternatives you've considered

Could also do logger.debug(() => ''), just a plain old function. But the templating idea seems pretty useful, at the end of the day it does in fact return a function too. So if you're going to template something, returning a function to be called might be faster if there are lots of logger.debug calls going around.

Additional context

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals#tagged_templates

Should benchmark if this really improves performance, but it also provides some usability... we might be more free to sprinkle logger.debug calls if they don't slow things down (except a function call).