JavaScript: Property Dot Notation vs Bracket Notation

There are 3 primary syntaxes to access a object's property:

object.key

Dot notation. Most convenient.

• Dot notation cannot be used when the property name contains space. For example, {"a b":7}
• Dot notation cannot be used when the property name is a language keyword or reserved word. For example, obj.for.
• Dot notation cannot be used when the property name is number. For example, obj.7
• Dot notation cannot be used to create new property at run time. For example, when you have a property name as value of a variable, constructed from user input.
• Dot notation cannot be used for Symbol key properties.

object[key]

Bracket notation. Useful if key contains space or is a number or Symbol type, or is a variable.

object?.key

Same as dot notation, but return undefined instead of error out if object is undefined or null [see Optional Chaining Operator ]

Example of a property name with space in it:

// property name that contains space
const 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
const x = { "3": 8 };

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

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

following are syntax errors
dot notation cannot be used

x."3"

SyntaxError: Unexpected string

x.3

SyntaxError: Unexpected number

Here is 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

const jj = { "p1": 1 };
const v = "p2";

jj[v] = 2;

console.log(jj); // { p1: 1, p2: 2 }

// all true
console.log(jj.v === undefined);
console.log(jj[v] === jj["p2"]);
console.log(jj["p2"] === 2);