I have a code implementation that iterates over an object's properties.

for (const prop in obj) {
    propsMap[prop] = prop;
}

But as is states, my IDE (WebStorm) adviced me to add a property check using obj.hasOwnProperty(prop) to avoid iterating over inexistant properties:

for (const prop in obj) {
    if (obj.hasOwnProperty(prop)) {
        propsMap[prop] = prop;
    }
}

The problem is that the current tests always e with obj.hasOwnProperty(prop) being true and the coverage is not the best I could get and I don't know what could happen if obj does not actually have the property prop.

I have a code implementation that iterates over an object's properties.

for (const prop in obj) {
    propsMap[prop] = prop;
}

But as is states, my IDE (WebStorm) adviced me to add a property check using obj.hasOwnProperty(prop) to avoid iterating over inexistant properties:

for (const prop in obj) {
    if (obj.hasOwnProperty(prop)) {
        propsMap[prop] = prop;
    }
}

The problem is that the current tests always e with obj.hasOwnProperty(prop) being true and the coverage is not the best I could get and I don't know what could happen if obj does not actually have the property prop.

• javascript
• jasmine
• code-coverage

• "what could happen if obj does not actually have the property prop". the object might inherit from another object via prototype. in this case those properties will be iterated but not "own". If you don't inherit from other object and just create your object with maybe var obj = {...}, then hasOwnProperty check is useless. – dfsq Commented Aug 10, 2017 at 6:34
• The object can e from anywhere because this is in a library I'm creating (it's typescript originally, but this is not a typescript-related issue so I didn't tag it as typescript). I think that the test isn't useless here, but I would like to know how I can create an object so that obj.hasOwnProperty(prop) will be false. – Supamiu Commented Aug 10, 2017 at 6:38
• How about Object.keys(obj).forEach(prop => { propsMap[prop] = prop })? Getting coverage on that would be much easier – Phil Commented Aug 10, 2017 at 6:44

2 Answers 2

To test this you could create object that inherits something from its prototype

const obj = Object.create({name: 'inherited'})

name will falsyfy obj.hasOwnProperty('name') check.

But there are better options to copy object. For example Object.assign

Object.assign(propsMap, obj)

Also you should keep in mind that obj.hasOwnProperty check is error prone. For example

const obj = {hasOwnProperty: null} // hasOwnProperty is not a function
const obj = Object.create(null) // obj wont inherit hasOwnProperty 

so atleast replace it with

const hasOwnProperty = {}.hasOwnProperty

for(const name in obj) {
  if(hasOwnProperty.call(obj, name)) {

  }

This worked for me:

const parentObj = {
   defaultProp: 'test'
}

const object = Object.create(parentObj, {
    prop: {
       value: 'test',
       enumerable: true
    }
}

The defaultProp property should return false in the hasOwnProperty check