mutate-cow

import mutate from 'mutate-cow';

const animals = deepFreeze({
  cats: ['ragamuffin', 'shorthair', 'maine coon'],
});

const newAnimals = mutate(animals)
  .set('dogs', ['hound'])
  .update('cats', (ctx) => {
    ctx.write().push('bobtail');
  })
  .final();

This module allows you to update an immutable object as if it were mutable. It has copy-on-write semantics, so properties are only changed if you write to them. (In fact, if you perform no writes, the same object is returned back.) This makes it useful in conjuction with libraries like React, where state may be compared by reference.

mutate-cow provides useful features that other packages don't:

• All property descriptors from the immutable object are preserved in the copy.
• All extensibility information from the immutable object is preserved in the copy. Combined with the above point, this means that sealed objects stay sealed and frozen objects stay frozen.
• Arrays, objects, and class instances are supported for mutation.
• Flow and TypeScript definitions are provided.

No cows were harmed in the making of this code.

API

const ctx = mutate(source)

Returns a "context" object which can modify a copy of source.

const foo = deepFreeze({bar: {baz: []}});
const ctx = mutate(foo);

ctx.read()

Returns the current working copy of the context's source object, or just source if no changes were made.

ctx.read() === foo; // no changes
ctx.set('bar', 'baz', ['qux']);
ctx.read().bar.baz[0] === 'qux'; // changes

ctx.write()

Returns the current working copy of the context's source object. Makes a shallow copy of source first if no changes were made.

You normally don't need to call write. It's mainly useful for accessing methods on copied objects (e.g., array methods).

ctx.get('bar', 'baz').write().push('qux');
ctx.read().bar.baz[0] === 'qux';

ctx.get(...path: [prop1, ...])

Returns a child context object for the given path.

Passing zero arguments returns ctx.

ctx.get() === ctx;
ctx.get('bar').read() === foo.bar;
ctx.get('bar', 'baz').read() === '';

ctx.set(...path: [prop1, ...], value)

Sets the given path to value on the current working copy. Returns ctx.

Passing zero property names (i.e., only a value) sets the current context's value.

// these all do the same thing
ctx.set({bar: {baz: 2}});
ctx.set('bar', {baz: 2});
ctx.set('bar', 'baz', 2);
ctx.get('bar').set({baz: 2});
ctx.get('bar').set('baz', 2);
ctx.get('bar', 'baz').set(2);

ctx.update(...path: [prop1, ...], updater)

Calls updater(ctx.get(...path)) and returns ctx.

const copy = ctx
  .update('bar', 'baz', (bazCtx) => {
    bazCtx.write().push('qux');
  })
  .final();
copy.bar.baz[0] === 'qux';

ctx.parent()

Returns the parent context of ctx.

ctx.parent() === null;
ctx.get('bar').parent() === ctx;
ctx.get('bar', 'baz').parent() === ctx.get('bar');

ctx.root()

Returns the root context of ctx.

ctx.root() === ctx;
ctx.get('bar').root() === ctx;
ctx.get('bar', 'baz').root() === ctx;

ctx.revoke()

Revokes ctx so that it can no longer be used. Returns undefined.

Attempting to use any method other than isRevoked on a revoked context will throw an error. This sets all internal properties to null so that there's no longer any reference to the source object or copy.

ctx.isRevoked()

Returns a boolean indicating whether ctx has been revoked.

ctx.final()

This is the same as read, except it also revokes the context and restores all property descriptors and extensibility information. This is what you call to get the final copy.

const copy = mutate(foo).set('bar', 'baz', 'qux').final();
Object.isFrozen(copy) === true; // since `foo` was frozen, `copy` will be too

ctx.finalRoot()

Returns ctx.root().final().