for in loop JavaScript returning undefined [duplicate]

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
– mousetail, Commented Aug 11, 2022 at 7:47
@mousetail key is the name of the for-loop variable, iterating on the keys of the dictionnary. — Benjamin Rio
– Benjamin Rio, Commented Aug 11, 2022 at 7:50
obj.key and obj["key"] are equivalent and not to be confused with obj[key]. — Ouroborus
– Ouroborus, Commented Aug 11, 2022 at 7:50
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
– Max Joy, Commented Aug 11, 2022 at 7:53

Benjamin Rio · Accepted Answer · 2022-08-11 07:54:18Z

1

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").

answered Aug 11, 2022 at 7:54

Benjamin Rio

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