JS: Property Dot Notation / Bracket Notation

By Xah Lee. Date: . Last updated: .

There are 2 syntax to access a object's property:

They are equivalent, except:

Example of a property name with space in it:

// property name containing space
var x = {"a b": 8};

console.log(
    x["a b"]
);          // 8

// the following are syntax errors

x[a b]
// SyntaxError: Unexpected identifier

x."a b"
// SyntaxError: Unexpected string

Example of a property name of digits:

// a property with name that's a digit
var x = {"3":8};

console.log(x["3"]); // 8

console.log(x[3]); // 8
// JavaScript automatically convert 3 to string

// dot notation cannot be used
x."3"
// SyntaxError: Unexpected string

// dot notation cannot be used
x.3
// SyntaxError: Unexpected number

Here's a example of creating property with a name from the value of a variable.

// creating a property, with name from a value of variable
// dot notation won't work

var obj = {"p1":1};
var v = "p2";
obj[v] = 2;
console.log(obj); // { p1: 1, p2: 2 }

JS Object Property Topic

  1. JS: Property Overview
  2. JS: Property Key
  3. JS: Property Dot Notation / Bracket Notation
  4. JS: Create/Delete Property
  5. JS: Get Property, Set Property
  6. JS: Check Property Existence
  7. JS: Access Property
  8. JS: Property Attributes, writable, enumerable, configurable
  9. JS: Getter/Setter Properties
  10. JS: Property Descriptor
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