What does Object.freeze do?

It makes an object's own properties read-only and prevents adding or deleting any — a one-way operation that cannot be undone.

const config = Object.freeze({ api: "/v1", retries: 3 });

config.api = "/v2";      // ignored silently (TypeError in strict mode)
config.timeout = 1000;   // ignored
delete config.retries;   // ignored
Object.isFrozen(config); // true

Note the silence. In sloppy mode the assignment simply does nothing, which is confusing during debugging; in strict mode and modules it throws, which is better.

It is shallow

This is the detail that catches everyone:

const settings = Object.freeze({ theme: { dark: true } });
settings.theme.dark = false;   // works — the nested object is not frozen

A deep freeze means recursing yourself:

function deepFreeze(obj) {
  for (const v of Object.values(obj)) {
    if (v && typeof v === "object") deepFreeze(v);
  }
  return Object.freeze(obj);
}

The related methods

Where it is genuinely useful

Configuration objects and constant lookup tables, where an accidental write is a real bug. Enforcing immutability in a shared module so no consumer can mutate exported state.

Where it is not

It is not a substitute for immutable update patterns. In React you do not freeze state — you create new objects with spread, and the benefit comes from the changed reference, which freezing does not give you.

There is also a small runtime cost, and it can deoptimise hot paths in some engines, so freezing large objects on every render is a bad trade. Freeze at module setup, not in a loop.