JavaScript Comparison and Logical Operators

Comparison and Logical operators are used to test for true or false.

Comparison Operators

Comparison operators are used in logical statements to determine equality or difference between variables or values.

Given that x = 5, the table below explains the comparison operators:

Operator Description Comparing Returns
== equal to x == 8 false
x == 5 true
x == "5" true
=== equal value and equal type x === 5 true
x === "5" false
!= not equal x != 8 true
!== not equal value or not equal type x !== 5 false
x !== "5" true
x !== 8 true
> greater than x > 8 false
< less than x < 8 true
>= greater than or equal to x >= 8 false
<= less than or equal to x <= 8 true

How Can it be Used

Comparison operators can be used in conditional statements to compare values and take action depending on the result:

if (age < 18) text = "Too young to buy alcohol";

You will learn more about the use of conditional statements in the next chapter of this tutorial.

Logical Operators

Logical operators are used to determine the logic between variables or values.

Given that x = 6 and y = 3, the table below explains the logical operators:

Operator Description Example
&& and (x < 10 && y > 1) is true
|| or (x == 5 || y == 5) is false
! not !(x == y) is true

Conditional (Ternary) Operator

JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.

Syntax

variablename = (condition) ? value1:value2 

Example

let voteable = (age < 18) ? "Too young":"Old enough";

If the variable age is a value below 18, the value of the variable voteable will be "Too young", otherwise the value of voteable will be "Old enough".

Comparing Different Types

Comparing data of different types may give unexpected results.

When comparing a string with a number, JavaScript will convert the string to a number when doing the comparison. An empty string converts to 0. A non-numeric string converts to NaN which is always false.

Case Value
2 < 12 true
2 < "12" true
2 < "John" false
2 > "John" false
2 == "John" false
"2" < "12" false
"2" > "12" true
"2" == "12" false

When comparing two strings, "2" will be greater than "12", because (alphabetically) 1 is less than 2.

To secure a proper result, variables should be converted to the proper type before comparison:

age = Number(age);
if (isNaN(age)) {
  voteable = "Input is not a number";
} else {
  voteable = (age < 18) ? "Too young" : "Old enough";
}

The Nullish Coalescing Operator (??)

The ?? operator returns the first argument if it is not nullish (null or undefined).

Otherwise it returns the second argument.

Example

let name = null;
let text = "missing";
let result = name ?? text;

The nullish operator is supported in all browsers since March 2020:

         
Chrome 80 Edge 80 Firefox 72 Safari 13.1 Opera 67
Feb 2020 Feb 2020 Jan 2020 Mar 2020 Mar 2020

The Optional Chaining Operator (?.)

The ?. operator returns undefined if an object is undefined or null (instead of throwing an error).

Example

// Create an object:
const car = {type:"Fiat", model:"500", color:"white"};
// Ask for car name:
document.getElementById("demo").innerHTML = car?.name;