Keeping data immutable in JavaScript

Copy, do not mutate — and know how deep the copy goes.

const state = {
  lang: "javascript",
  runs: [104, 98],
  meta: { updated: "2026-07-30" },
};

const shallow = { ...state, runs: [...state.runs, 91] };
shallow.meta.updated = "changed"; // shared with state
console.log(state.meta.updated, shallow.runs);

const deep = structuredClone(state);
deep.meta.updated = "isolated";
console.log(state.meta.updated, deep.meta.updated);

const frozen = Object.freeze({ nested: { a: 1 } });
frozen.nested.a = 2; // allowed: freeze is shallow
console.log(Object.isFrozen(frozen), frozen.nested.a);

How it works

1. Spread and the to* methods copy one level.
2. structuredClone copies deeply, including Maps and Dates.
3. Object.freeze stops writes, but only at the top level.

Keywords and builtins used here

• Object
• const

The run, in numbers

Lines

17

Characters to type

529

Tokens

147

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 80 seconds.

Type this snippet

Step 4 of 4 in Objects under the hood, step 15 of 16 in Functions & patterns.

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