Chrome Uncaught Type Error: Cannot read property undefined

Question

I’m a complete novice to programming and am working my way through the book Learning to Program, by Steven Foote. I am trying to find out why Chrome is telling me I’ve got these two errors:

Uncaught TypeError: Cannot read property ‘currentDate’ of undefined
Uncaught TypeError: Cannot read property ‘projectName’ of undefined

This is what I've written:

values.js

var kbValues = {
 projectName: 'kittenbook',
 versionNumber: '0.0.1',
 currentDate: new Date(), 
 currentTime: [kbValues.currentDate.getFullYear() + '-' + 
(kbValues.currentDate.getMonth() + 1)+ '-' +
kbValues.currentDate.getDate() + ' at ' + 
kbValues.currentDate.getHours() + ':' + 
kbValues.currentDate.getMinutes() + ':' + 
kbValues.currentDate.getSeconds()]
};

kittenbook.js

document.body.innerHTML = '<h1>Hello, ' + userName + '!</h1>' + 
'<p>' + kbValues.projectName + '' + kbValues.versionNumber +
' accessed on: ' + kbValues.currentTime + '</p>';

manifest.json

{
    "manifest_version": 2,
    "name": "kittenbook",
    "description": "Replace photos on Facebook with kittens",
    "version": "0.0.1",
    "content_scripts": [
        {
            "matches": ["*://www.facebook.com/*"],
            "js": ["js/values.js","js/kittenbook.js"]
        }
    ]
}

I'm really very new to this.

@GetSet We all start somewhere. If not here, where do you suggest a beginner ask for help? — mizliz
– mizliz, Commented Feb 19, 2020 at 11:42

Krzysztof Krzeszewski · Accepted Answer · 2020-02-19 11:02:31Z

you cant use properties of a variable during its definition, you are trying to access currentDate of kbValues, before assigning it

As you can see this snippet will return an error:

var kbValues = {
 projectName: 'kittenbook',
 versionNumber: '0.0.1',
 currentDate: new Date(), 
 currentTime: [kbValues.currentDate.getFullYear() + '-' + 
  (kbValues.currentDate.getMonth() + 1)+ '-' +
   kbValues.currentDate.getDate() + ' at ' + 
   kbValues.currentDate.getHours() + ':' + 
   kbValues.currentDate.getMinutes() + ':' + 
   kbValues.currentDate.getSeconds()]
};

instead you should try assigning the date before variable definition, in this way you will be able to access it

var currentDate = new Date();

var kbValues = {
 projectName: 'kittenbook',
 versionNumber: '0.0.1',
 currentDate, 
 currentTime: [currentDate.getFullYear() + '-' + 
  (currentDate.getMonth() + 1)+ '-' +
   currentDate.getDate() + ' at ' + 
   currentDate.getHours() + ':' + 
   currentDate.getMinutes() + ':' + 
   currentDate.getSeconds()]
};

console.log(kbValues)

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