copy-with

copy-with

A library for immutably working with JavaScript objects.

TL;DR

$ npm install --save copy-with

var cw = require('copy-with');
var foo = { x: 1, y: 1 };
var bar = cw.copyWith(foo, { y: 2, z: 3 }); // { x: 1, y: 2, z: 3 }

Principles and Goals

• Pure functions (never mutate input objects)
• Make it easy to "modify" immutable objects, or objects you want to treat as immutable
• Don't wrap objects or add methods to them - use plain JavaScript objects
  • (that means that output objects will work anywhere)
• If an output object would be identical to the input, just return the input.

  • This preserves referential equality and allows comparisons like this:

    if (prevObj === newObj) { /* `newObj` has definitely changed */ }
    

  • This works very nicely with Redux, among other functionally-inspired libraries and patterns!

API

cw.copyWith(input, changes)

Returns a copy of input with changes applied. Similar to Object.assign({}, input, changes), but will return the original input if applying changes wouldn't actually change any values.

Examples:

copyWith({ x: 1 }, { y: 2 }) // { x: 1, y: 2 }
copyWith({ x: 1 }, { x: 2 }) // { x: 2 }
copyWith({ x: 1, y: 2 }, { x: 1 }) // Returns `input`, no changes necessary

cw.copyWithout(input, ...keys)

Returns a copy of input with keys (and their associated values) removed, or returns the original input if it does not have any of the keys specified.

Examples:

copyWithout({ x: 1, y: 2 }, 'y') // { x: 1 }
copyWithout({ x: 1, y: 2, z: 3 }, 'x', 'y') // { z: 3 }
copyWithout({ x: 1 }, 'y') // Returns `input`, no changes necessary

cw.copyWithVal(input, key, value)

Returns a copy of input with a single change made: key is set to value. Returns the original input if key is already set to value in input.

Examples:

copyWithVal({ x: 1, y: 2 }, 'y', 3) // { x: 1, y: 3 }
copyWithVal({ x: 1, y: 2 }, 'x', 1) // Returns `input`, no changes necessary

cw.copyWithDeep(input, path, changes)

Returns a copy of input where changes have been applied to the deeply nested object at path, which is an array of string keys. Returns the original input if there are no changes to be made.

Examples:

copyWithDeep({ x: { y: { z: 1 } } }, ['x', 'y'], { w: 2 }) // { x: { y: { z: 1, w: 2} } }
copyWithDeep({ x: { y: 1, z: 2 } }, ['x'], { y: 1 }) // Returns `input`, no changes necessary

cw.copyWithDeepVal(input, path, value)

Returns a copy of input where the deeply nested property at path (which is an array of string keys) has been set to value. Returns the original input if there is no change to be made.

Examples:

copyWithDeepVal({ x: { y: { z: 1 } } }, ['x', 'y', 'z'], 2) // { x: { y: { z: 2 } } }
copyWithDeepVal({ x: { y: 1 } }, ['x', 'y'], 1) // Returns `input`, no changes necessary

cw.copyWithDeepFn(input, path, fn)

Returns a copy of input where the deeply nested property at path (which is an array of string keys) has been set to the return value of calling fn with the current value. Useful for situations where you want to modify a value rather than replacing it, which would otherwise require listing the path twice.

Examples:

copyWithDeepFn({ x: { y: { z: 1 } } }, ['x', 'y', 'z'], z => z + 1) // { x: { y: { z: 2 } } }
copyWithDeepFn({ x: { y: { z: [1, 2] } } }, ['x', 'y', 'z'], z => z.concat([3, 4])) // { x: { y: { z: [1, 2, 3, 4] } } }

cw.firstIfSame(original, modified)

Returns modified, unless it is identical (given a shallow equality comparison) to original, in which case it returns original. This function supports arrays, unlike most functions in this library!

Examples:

var array = ['one', 'two', 'three']
firstIfSame(array, array.map(s => s.toUpperCase())) // Returns `modified`, which is ['ONE', 'TWO', 'THREE']
firstIfSame(array, array.map(s => s.toLowerCase())) // Returns `original`, which is `array` (['one', 'two', 'three'])

Development and Contributing

You'll need to be running at least Node v6 - this project is written in ES2015 (aka ES6) and doesn't use Babel for tests.

Clone this repo, and run npm install. You can run npm test to run unit tests. Run npm run build to compile to ES5 in the lib directory.

Publishing workflow

$ npm test
$ npm version [major|minor|patch]
$ npm run build
$ npm publish
$ git push --tags

Wishlist

We'd gladly accept PRs for the following:

• Support testing and building on Windows (should just be matter of using cross-env and a few other cross-platform scripts)
• Support ES3 environments (right now there's a dependency on many of the ES5 Array.prototype methods)
• Support arrays, especially in the copyWithDeep and copyWithDeepVal functions
• UMD build for non-CommonJS environments