smart-copy

A shortcut for copying properties to new objects with a little smarts:

• Easily specify defaults
• Prevent copying unwanted properties

Branch | Status ------ | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Master | | g All |

Usage

Install the library,

npm install --save smart-copy

Copy Properties from a Data Object

const smartCopy = require('smart-copy')

// The object we're copying to
const target = {}

// Definition of allowable properties, and their defaults
const properties = {
  enabled: true,
  name: undefined  // use `undefined` for properties with no meaningful default
}

// Data to be copied
const data = {
  name: 'Jimmy Bond',
  secretAgent: true  // won't be copied, since no entry exists in `properties`
}

smartCopy(target, properties, data)

// defaulted to true, as set in the props object
t.is(target.enabled, true)

// copied from the data, since it matches an entry in the properties
t.is(target.name, 'Jimmy Bond')

// not copied from data, since no property is defined
t.false(target.hasOwnProperty('secretAgent'))

Create a Class with a (Smart) Copy Constructor

A common pattern is to create a class that accepts defined data. This is easy by using smart-copy from the constructor:

const smartCopy = require('smart-copy')

// Definition of allowable properties, and their defaults
const properties = {
  enabled: true,
  name: undefined  // use `undefined` for properties with no meaningful default
}

// A class with a (smart) copy constructor
class Target {
  constructor(data) {
    smartCopy(this, properties, data)
  }
}

// Create a new instance with data
const target = new Target({
  name: 'Jimmy Bond',
  secretAgent: true  // won't be copied, since no entry exists in `properties`
})

// defaulted to true, as set in the props object
t.is(target.enabled, true)

// copied from the data, since it matches an entry in the properties
t.is(target.name, 'Jimmy Bond')

// not copied from data, since no property is defined
t.false(target.hasOwnProperty('secretAgent'))

API

smartCopy(target, properties, data)

Copy key-value pairs from data defined in the properties to the target.

Remember our goals:

• Easily specify defaults
• Prevent copying unwanted properties

The properties object acts as a list of both defaults and allowed entries. If a key exists in the properties but does not exist in the data, it will use the value from properties as a default. If a key is not defined in properties, it will not be copied from the data. This prevents accidentally or maliciously copying entries you might not want in the target object.

If an entry doesn't have a meaningful default, set the property to undefined. This will let smart-copy know that it is allowed, and will set it to undefined if nothing is passed in the data.

This module uses Object.hasOwnProperty(...) to determine if a property exists rather than just checking for undefined values, so that you can explicitly used undefined as a value in both properties and data.