issue accessing JSON object using key name with . but works with []

Question

What is the reason here while accessing JSON object using . gives error but using [] it works? what is the correct syntax to access the JSON object key?

This syntax gives the error:

var obj2G = obj.2G;

This syntax is working but why need to access this way?

var obj2G = obj["2G"];

var obj = {
  "2G": [{
    "essid": "SINGTEL-662F",
    "authmode": "psk psk2",
    "authkey": "0000026159",
    "isEnable": "1",
    "isHidden": "0",
    "hwaddr": "E0:8E:3C:00:66:30",
    "opmode": "ap"
  }, {
    "essid": "GUEST1-662F",
    "authmode": "psk psk2",
    "authkey": "aabbccddee",
    "isEnable": "0",
    "isHidden": "0",
    "hwaddr": "",
    "opmode": "ap"
  }, {
    "essid": "GUEST2-662F",
    "authmode": "psk psk2",
    "authkey": "aabbccddee",
    "isEnable": "0",
    "isHidden": "0",
    "hwaddr": "",
    "opmode": "ap"
  }, {
    "essid": "GUEST3-662F",
    "authmode": "psk psk2",
    "authkey": "aabbccddee",
    "isEnable": "0",
    "isHidden": "0",
    "hwaddr": "62:8E:3C:00:66:33",
    "opmode": "ap"
  }],
  "5G": [{
    "essid": "SINGTEL-662F(5G)",
    "authmode": "psk psk2",
    "authkey": "0000026159",
    "isEnable": "1",
    "isHidden": "0",
    "hwaddr": "E0:8E:3C:00:66:31",
    "opmode": "ap"
  }, {
    "essid": "GUEST1(5G)-662F",
    "authmode": "psk psk2",
    "authkey": "aabbccddee",
    "isEnable": "0",
    "isHidden": "0",
    "hwaddr": "6a:8e:3c:00:66:32",
    "opmode": "ap"
  }, {
    "essid": "GUEST2(5G)-662F",
    "authmode": "psk psk2",
    "authkey": "aabbccddee",
    "isEnable": "0",
    "isHidden": "0",
    "hwaddr": "",
    "opmode": "ap"
  }, {
    "essid": "GUEST3(5G)-662F",
    "authmode": "psk psk2",
    "authkey": "aabbccddee",
    "isEnable": "0",
    "isHidden": "0",
    "hwaddr": "6a:8e:3c:00:66:30",
    "opmode": "ap"
  }]
}

obj2G = obj["2G"];
console.log(obj2G);

obj2G = obj.2G;
console.log(obj2G);

http://stackoverflow.com/questions/1661197/what-characters-are-valid-for-javascript-variable-names — Satpal, Jun 13 '16 at 07:24

Azamantes · Answer 1 · 2016-06-13T07:30:59.697

2

Object properties must start with a letter when used in dot notation. It's just a rule in Javascript. You can name property literally '*&^' but you have to do it via [] notation.

2G starts with a number thus requires [] notation.

edited Jun 13 '16 at 07:30

answered Jun 13 '16 at 07:22

Azamantes

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15

i understood overall but can't understood what you wrote about `'*&^'` ? – Amit Shah Jun 13 '16 at 07:31
I mean you can add a property named `*&^` to an object like `object['*&^'] = 6;` – Azamantes Jun 13 '16 at 07:33
Found an article [related to this](https://mathiasbynens.be/notes/javascript-identifiers-es6) – Rajesh Jun 13 '16 at 07:38

Gerardo Furtado · Accepted Answer · 2016-06-13T08:53:56.337

If you use dot notation:

property must be a valid JavaScript identifier, i.e. a sequence of alphanumerical characters, also including the underscore ("_") and dollar sign ("$"), that cannot start with a number. For example, object.$1 is valid, while object.1 is not.

In that case, you have to use bracket notation:

property_name is a string. The string does not have to be a valid identifier; it can have any value, e.g. "1foo", "!bar!", or even " " (a space).

See the documentation: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Property_accessors

It's worth mentioning that you have to use bracket notation if you want to access a property using a variable. For instance:

var myName = "foo";
var myObject = { foo: 42};
console.log(myObject.myName);//returns undefined

But:

var myName = "foo";
var myObject = { foo: 42};
console.log(myObject[myName]);//returns 42

score 1 · Answer 3 · answered Jun 13 '16 at 07:25

From the documentation to Property Accessor for dot notation:

In this code, property must be a valid JavaScript identifier, i.e. a sequence of alphanumerical characters, also including the underscore ("_") and dollar sign ("$"), that cannot start with a number. For example, object.$1 is valid, while object.1 is not.

And to Variable

A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase).

The result is, a variable or a property in dot notation with starting number can not be used.

issue accessing JSON object using key name with . but works with []

3 Answers3