Immer

Requirements

• Export produce(base, recipe) as named and default. The synchronous recipe receives an independently mutable draft of base. Input is an acyclic tree of primitives, ordinary/null-prototype objects, and dense arrays containing own enumerable string-keyed data properties. No aliases, accessors, symbols, custom array properties, classes, or special built-ins are required.
• Allow nested property assignment, deletion, and ordinary array operations such as push/splice. Never mutate base, including when base is frozen. When the recipe returns undefined or the root draft, use the draft edits as the result. A primitive draft cannot be mutated; undefined return leaves that primitive unchanged.
• Preserve structural sharing: unchanged draft containers resolve to their original containers, and changed containers are fresh. Compare final own keys, array length, and values using Object.is. Setting an existing field to the same value or editing then restoring it is a no-op. Adding an own undefined property is a change.
• A recipe may return a replacement instead of editing. Return the exact primitive or external container replacement when it contains no draft references. Finalize draft references embedded in new containers. Throw TypeError if the final draft differs from base and a distinct non-undefined replacement is also returned. Returning root draft is allowed after edits.
• Preserve object prototypes and safely define arbitrary own keys, including __proto__. Drafts and final changed containers have writable data properties. Do not freeze the result automatically. Return thrown recipe values unchanged; never commit a partial result after an exception.
• Reject non-function recipes and recipes returning a thenable with TypeError. Async recipes, retained draft access after the call, cycles introduced by a recipe, getters/proxy traps, patch generation, Maps/Sets, currying, and replacing state with undefined are excluded. You may use eager cloning plus reconciliation or copy-on-write proxies; proxy optimization is not required.

Examples

Input: const base={user:{score:1},settings:{dark:true}}; const next=produce(base,draft=>{draft.user.score++;}); [next.user.score,next.settings===base.settings,base.user.score]
Output: [2,true,1]

Input: produce([1,2],draft=>{draft.push(3);})
Output: [1,2,3]