DRY principle

Background

We want to have a function which checks whether an item in JS object representation is a commondity. An item is a commondity if it satisfies one of :

• HAS a price attribute
• HAS a barcode attribute

The following tests should pass:

isCommondity({price: 1}); //true
isCommondity({barcode: 'abc'}); //true
isCommondity({a: 'a'}); //false

Round one

It seems to be quite straightfordword to come out with the following solution:

function isCommondityV1(comd) {
  return !!comd.price || !!comd.barcode
}

The implementation is wrong under some circumstances. !!obj.prop is checking the truthy of the prop property of obj , instead of checking the existence of the prop property. E.g.

isCommondityV1({price: 0}); // false

while we expect isCommondityV1 to return true since {price: 0} does have the price property.

Round two

hasOwnProperty

function isCommondityV2(comd) {
  return comd.hasOwnProperty('price') || comd.hasOwnProperty('barcode')
}

isCommondityV2({price: 0}); //true

The solution works perfectly in line with the functionality requirement. However, it does suffer from the two problems:

• Flexibility: if we add more checkings later on, we would have to update the implementation body. E.g. when the attribute set extends to be ['price', 'barcode', 'a', 'b'] .

function isCommondityV21(comd) {
  return comd.hasOwnProperty('price') || 
          comd.hasOwnProperty('barcode') || 
          comd.hasOwnProperty('a') || 
          comd.hasOwnProperty('b');
}

• DRY: We're repeating the operation (i.e comd.hasOwnProperty ). We can be better off by seperating the variables and encapsulate the constants.

Round three

function isCommondityV3(comd) {
  return ['price', 'barcode'].some(Object.prototype.hasOwnProperty, comd);
}

With this implementation, we can easily deal with later change by updating the variables.

function isCommondityV31(comd) {
  return ['price', 'barcode', 'a', 'b'].some(Object.prototype.hasOwnProperty, comd);
}

Or we can even go one more step further by encapsulating the variables into config file. Futher requirement change will result in only changes in config file without touching the implementation body :-)

//config.js
export COMMONDITY_PROPS = ['price', 'barcode'];

//commondity.js
import { COMMONDITY_PROPS } from 'config.js'

function isCommondity(comd) {
  return COMMONDITY_PROPS.some(Object.prototype.hasOwnProperty, comd);
}

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