Immutable JavaScript object updater (simple implementation)

immutableUpdate.js

import equal from 'deep-equal'

export function immutableUpdate( obj, change ) {
    const keys = Object.keys( change )
    if ( keys.length === 0 ) {
        return obj
    }
    const key = keys[ 0 ]
    const value = change[ key ]
    if ( keys.length === 1 ) {
        if ( !key ) {
            return obj
        }
        if ( !obj ) {
            return { [ key ]: value }
        }
        // if value is undefined delete it if it exists
        if ( value === undefined ) {
            const newObj = Object.assign( {}, obj )
            delete newObj[ key ]
            return newObj
        }
        const current = obj[ key ]
        if ( current && equal( current, value ) ) {
            return obj
        }
        return Object.assign( {}, obj, { [ key ]: value } )
    }
    const tail = Object.assign( {}, change )
    delete tail[ key ]
    // TODO: fold/reduce/make tail recursive
    return updateObject( updateObject( obj, { [ key ]: value } ), tail )
}

export function updateAtom( current, update ) {
    if ( update === undefined ) { return current }
    return equal( current, update ) ? current : update
}