Frozen objects

JavaScript · Unit 15: Immutability

All of this depends on nobody forgetting to copy. Object.freeze(obj) puts the lock on: the object becomes read-only.

In the mode your code runs in, assigning to a frozen property crashes with TypeError.

const menu = Object.freeze({
  dish: "soup",
});
try {
  menu.dish = "stew";
} catch (e) {
  console.log(e.name);
}
console.log(menu.dish);

Prints

TypeError
soup

The rest of the explanation is in the lesson, which is part of the full plan.

Exercises in this lesson

You do them in the app, which checks them on the spot and explains why.

1. Predict the output
The config is frozen and someone tries to add a property. What does it print?
2. Multiple choice
How are const and Object.freeze different?
3. Predict the output
An array gets frozen and then a push. What does it print?
4. Complete the code
Fill in the blank so the config is protected and the 99 doesn't get in.
5. Predict the output
The team is frozen, yet a player gets signed. What does it print?
6. Find the bug
This program should print a fresh config with max at 50, without touching the frozen one. Which line has the error?
7. Put the lines in order
Put the program in order: it tries to raise the max of a frozen config and, instead of crashing, prints the name of the error and the value that stayed.
8. Complete the code
Fill in the blank so raise returns the max plus 5, without touching the frozen config.
9. Find the case that fails
adjust(conf, n) returns a fresh config with max at n, without changing the one it receives; to make sure, it freezes it on the way in. Which call does it fail on?
10. Predict the output
Two ways of sorting the same frozen array. What does it print?

It opens in your browser. This lesson is part of the full plan; the first unit of each course is free.