interpolation.js

String.prototype.interp = function() { 
  var matcher = /#{(.+?)}/g
  var replacer = function(_, expr) {
    return eval(expr)
  }

  return this.replace(matcher, replacer)
}

// Example:

var name = "Bob"
var age = 20

console.log("Your name is #{name}, and you are #{age} years old! (You were born in #{new Date().getFullYear() - age}!)".interp())