object.key value is showing undefined in the for in loop?

Question

as u can see below , obj.key outside the for in loop is working properly but why not in the for in loop ?

and obj[key] inside the for in loop is working properly , what`s the reason behind one is working correctly and other one is not ?

code

let obj = {
  name: "lucky",
  hobbies: "coding"
}
console.log(obj.name)
for (let key in obj) {
  console.log(obj.key)
}

output

lucky
undefined
undefined

Does this answer your question? [Add a property to a JavaScript object using a variable as the name?](https://stackoverflow.com/questions/695050/add-a-property-to-a-javascript-object-using-a-variable-as-the-name) — Ivar, Jul 06 '22 at 07:45
@xupitan yeah thats y i mentioned that too , but i am asking why object.key is not working — lucky, Jul 06 '22 at 07:47
Because `key` is not the _name_ of a key and can't be accessed like a named key. — Andy, Jul 06 '22 at 07:48
because you are fetching the property name called 'key' from the object inside the obj variable. While if you want to fetch a property using a dynamic name given from a string you need to use the accessor `[]` https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Property_accessors — Diego D, Jul 06 '22 at 07:48
@andy it is through let key in obj , each time it iterates it assigns keys from the object right — lucky, Jul 06 '22 at 07:53
@ivar that is adding a property right and this is retrieving a property — lucky, Jul 06 '22 at 07:55
@lucky The dot is a property accessor that looks for a literal `key` in your object. Your object contains a `name` and `hobbies` key, not a `key` key. The bracket notation allows you to access a property using a JavaScript value such as the string the `key` variable evaluates to. — Ivar, Jul 06 '22 at 07:57
Alternative duplicate: [JavaScript property access: dot notation vs. brackets?](https://stackoverflow.com/questions/4968406/javascript-property-access-dot-notation-vs-brackets) — Ivar, Jul 06 '22 at 07:58
Because that's what the JavaScript Language architects decided @lucky. There really isn't much more to it. — Ivar, Jul 06 '22 at 08:00
@lucky it's a _variable_ that's been assigned the value of the current key in the iteration. "key" is not a named key in the object and, therefore, you need to use `obj[key]` to access the value. Read that second dupe that @Ivar added. — Andy, Jul 06 '22 at 08:02
But you shouldn't when there's a far simpler method that doesn't involve `eval`. — Andy, Jul 06 '22 at 08:07

score 1 · Answer 1 · answered Jan 02 '23 at 10:47

For accessing object if you using dot operator then you have to give the key directly and you can not use variable name for that. eg => obj.name but you can not use obj.key as it conside key as a key name. You can user [] operator to access the key in the for loop. Thus your code will be =>

let obj = {
  name: "lucky",
  hobbies: "coding"
}
for (let key in obj) {
  console.log(obj[key])
}

object.key value is showing undefined in the for in loop?

1 Answers1