for in loop JavaScript returning undefined

Question

let obj = {
  a:10,
  b:20,
  c:true,
  d:null
}

for(let key in obj) {

  //console.log(key);

  //console.log(obj.key); //4 undefined

  console.log(obj[key]) // 10 20 true null //work
}

why dot notation giving me undefined.

I`m newbie started learning js.

Because there is no key named "key" on the object, the options are `a`, `b`, `c`, and `d` but not `key` — mousetail, Aug 11 '22 at 07:47
@mousetail key is the name of the for-loop variable, iterating on the keys of the dictionnary. — Benjamin Rio, Aug 11 '22 at 07:50
`obj.key` and `obj["key"]` are equivalent and not to be confused with `obj[key]`. — Ouroborus, Aug 11 '22 at 07:50
but key has a,b,c, and d values, I was expecting that it would be obj.a, obj.b, obj.c and obj.d. I`m I expecting wrong? — Max Joy, Aug 11 '22 at 07:53
@MaxJoy — Yes, as all the previous comments have said, you are asking for `obj.key` not `obj.a`. — Quentin, Aug 11 '22 at 07:55

score 1 · Answer 1 · answered Aug 11 '22 at 07:54

The for-loop iterates over the dictionnary keys. Therefore, key is a string.

You can access value of key a element using obj["a"] (notice the apostrophes) or using obj.a(notice the absence of the apostrophes).

Since key is a string obj.key does not work (in the first iteration it computes obj."a").

for in loop JavaScript returning undefined

1 Answers1