Key -> value mapping that keeps track of if any value has been updated.

TrackingMap.js

/**
 * Key -> value mapping that keeps track of if any value has been updated.
 */
class TrackingMap {
    constructor(initial={}) {
        this.updated = true

        for (let field in initial) {
            if (field.charAt(0) == '_') {
                throw new Error('You may not start field names with an' +
                    ' underscore.')
            }
            if (field == 'updated') {
                throw new Error('"updated" is a reserved field name')
            }

            let internalField = `_${field}`

            let initialValue = initial[field]
            this[internalField] = initialValue

            Object.defineProperty(this, field, {
                get: () => {
                    return this[internalField]
                },
                set: (value) => {
                    let before = this[internalField]
                    if (before !== value) {
                        this.updated = true
                    }
                    this[internalField] = value
                },
                configurable: true,
                enumerable: true
            })
        }
    }
}