DOT & BRACKET Notation in JavaScript

In JavaScript we can access the properties of object using dot notation(person.first_name) and bracket notation(person[first_name]). But what's the difference between these two notations and when to use which one ? Just stay here with me for another 20 mins and I promise you will never forget this concept.

First we will see what's the issue we will face using dot operator.

    let person = {
    	fname:'John',
    	lname:'Doe'
    }
    person.fname // John
    person["fname"] // John

    person[fname] //ReferenceError: fname is not defined
    person."fname" //SyntaxError: Unexpected string

Now,

let first_name = "fname";
person.first_name; // undefined
person[first_name]; // John

Why is it soooooo...... I hope you got at least 2% of clarity from this example !!

lets see one more example and clarification, when to use dot and when not too.

let obj = {
  cat: "meow",
  dog: "woof",
};
let dog = "cat";
console.log(obj.dog); // woof (not as expected ?)

let obj = {
  cat: "meow",
  dog: "woof",
};
let dog = "cat";
console.log(obj[dog]); // meow (the expected result)

In the above example, obj.dog does not evaluate the dog property and it directly denotes the dog property of the obj object, if dog property is present then it return the value otherwise undefined.

in obj[dog], here dog is first evaluated to get the property name. Upon evaluating the value of dog i.e cat is assigned and the final result is obj["cat"] and this will return "meow"

Why are we looking into these many comparisons ?

Sometimes when we are working in production with some API calls and with JSON data, then their might be the chance to get the key name with spaces, (& that we might don't know being a frontend developer) then in that case we cannot access it with dot notation. eg.

let obj = {
	"first name" : "John"
}

obj.first name //error
obj['first name"] // John

Lets see some of the important points of dot and bracket notations

(obj.x <---- here x is a property identifier)

To access javascript object properties using dot notation, the

Property identifiers can only be alphanumeric (and _ and $)
Property identifiers cannot start with a number.
Property identifiers cannot contain variables.
Valid Identifier (contain Unicode letters, $, _, and digits (0-9), but may not start with a digit)
do not start with a digit or hyphen
no spaces
do not include special characters except $ and _ (underscore)
obj.prop_1, obj.prop$ is correct
obj.1prop, obj.prop name is incorrect

To access javascript object properties using bracket notation, the key should be

Property identifiers have to be a String or a variable that references a String.
we can use variables, spaces, and Strings that start with numbers
it can be an expression
obj["1prop"], obj["prop name"] is correct

	let temp = "fav"
    let x=2;
    let y=3;
    let obj = {
    	[temp+"game"] : "cricket",
    	[temp] : 'coffee',
    	[x+y]: "some_value"
    }
    obj.temp //undefined
    obj[temp] //coffee
    obj.x+y //NaN
    obj.5 //syntax error
    obj[x+y] // "some value"
    obj.favgame // "cricket"
    obj[temp+"game"] // "cricket"

In Simple words, The two most common ways to access properties in JavaScript are with a dot and with square brackets. Both value.x and value[x] access a property on value—but not necessarily the same property. The difference is in how x is interpreted. When using a dot, the part after the dot must be a valid variable name, and it directly names the property. When using square brackets, the expression between the brackets is evaluated to get the property name. Whereas value.x fetches the property of value named “x”, value[x] tries to evaluate the expression x and uses the result as the property name.

Lets see how to assign variable as key of an object in JavaScript

let x = "name";
let obj = {
  x: "John",
};

console.log(obj.name); //undefined
console.log(obj.x); //John

Why we are getting undefined ? because we have not assigned the variable as the key of an object in proper way. To assign variable as key we have to use bracket notation [] as below,

let obj = {
  [x]: "John",
};
console.log(obj.x); //undefined
console.log(obj.name); //John

Dot Notation’s Limitation

if we take this object :

const obj = {
  123: 'digit',
  123name: 'start with digit',
  name123: 'does not start with digit',
  $name: '$ sign',
  name-123: 'hyphen',
  NAME: 'upper case',
  name: 'lower case'
};
//source: stackoverflow

accessing their proprieties using dot notation

obj.123;      // ❌ SyntaxError
obj.123name;  // ❌ SyntaxError
obj.name123;  // ✅ 'does not start with digit'
obj.$name;    // ✅  '$ sign'
obj.name-123;  // ❌ SyntaxError
obj.'name-123';// ❌ SyntaxError
obj.NAME; // ✅ 'upper case'
obj.name; // ✅ 'lower case'

But none of this is a problem for the Bracket Notation:

obj["123"]; // ✅ 'digit'
obj["123name"]; // ✅ 'start with digit'
obj["name123"]; // ✅ 'does not start with digit'
obj["$name"]; // ✅ '$ sign'
obj["name-123"]; // ✅ 'does not start with digit'
obj["NAME"]; // ✅ 'upper case'
obj["name"]; // ✅ 'lower case'

Some more mind blowing examples of dot and bracket notation in JS object

Example 1

let obj = {
  true: "john",
};
obj[true]; //john
obj[true]; //john
obj[!false]; //john
obj[!0]; //john
obj[1]; //undefined

let obj = {
  undefined: "john",
};
obj.undefined; //john
obj[undefined]; //john
obj[obj.x]; //john


2. Example 2
let obj={
  fname:"capscode"
}

let name={
  myname:"fname"
}
console.log(obj.name.myname)

What I want to do here is whatever value name.myname returns I will again use that value as a key in obj object. Can you guess the output here, please take your time and think about the output

the output of the above code is ReferenceError:cannot read property of undefined (reading 'myname')

NOTE: Dot notation obj.property requires a literal property name and doesn't support dynamic key access. Therefore, to dynamically access properties, bracket notation obj[propertyName] is necessary.

console.log(obj[name.myname]) //capscode
console.log(obj[name[myname]]) //ReferenceError:myname is not defined (check article why this error)
console.log(obj[name['myname']]) //capscode

Thank you for reading this far. This is a brief introduction to the Difference between accessing object properties using dot and bracket notation in JS. If you find this article useful, like and share this article. Someone could find it useful too. If you find anything technically inaccurate please feel free to comment below. Now you can also play around the objects in JS.

Hope it's a nice and informative read for you. VISIT https://www.capscode.in/blog TO LEARN MORE...