Truly Constant Props in Javascript

You can make mere variables and even an object immutable. But, how about a humble object prop?

Making object props constant and immutable has practical usage. For e.g. consider this totally practical program to print out fruits and whether they are poisonous.

const eatable = {
  poisonous: false,
  printFruit: function(fruit) {
    return "name: " + fruit + " | poison: " + this.poisonous;
  }
};

console.log(eatable.printFruit("mango")); // name: mango | poison: false

If someone decides to play spoil-sport and update poisonous to true, it may result in whole populations being wiped out.

// ..blah

eatable.poisonous = true;
console.log(eatable.printFruit("apple")); // name: apple | poison: true

We cannot afford to change constants midway. It turns out that the solution is simple - all we needed was to keep a constant as true constant by defining it as unwritable.

const eatable = {
  printFruit: function(fruit) {
    return "name: " + fruit + " | poison: " + this.poisonous;
  }
};

Object.defineProperty(eatable, "poisonous", {
  value: false,
  writable: false
});

Now, the logic is bulletproof and cannot unknowingly cause harm to a human.

console.log(eatable.printFruit("mango")); // name: mango | poison: false

eatable.poisonous = true;
// no error when not in strict mode
//   .. but constant does not change
console.log(eatable.printFruit("apple")); // name: apple | poison: false