03 August 2019

Check If Three Values are Equal in JavaScript

Equality Operators

In JavaScript, we can compare three variables to check if they are equal using the equality operator (==) or the strict equality operator (===). I've discussed the differences between the two in more detail before but as a quick recap: the equality operator compares values after type coercion, whilst the strict equality operator compares both value and type.

For example, let's say we have three variables a, b, and c and we want to check if they are equal:

let a = 1;let b = '1';let c = 1;if (a == b && b == c) {  console.log('a, b, and c are equal');} else {  console.log('a, b, and c are not equal');}//=> 'a, b, and c are equal'

In this example, a and c are both numbers, whilst b is a string. The equality operator will compare the values of a and b after converting the string '1' to a number, so the comparison will return true and the output will be "a, b, and c are equal".

If we want to compare the variables using the strict equality operator instead, it would look like this:

let a = 1;let b = '1';let c = 1;if (a === b && b === c) {  console.log('a, b, and c are equal');} else {  console.log('a, b, and c are not equal');}//=> 'a, b, and c are not equal'

In this case, the comparison will return false because a and b have different types, so the output will be "a, b, and c are not equal".

Using a Ternary

We could also combine our equality operators with a ternary operator:

let a = 1;let b = '1';let c = 1;console.log(  a === b && b === c ? 'a, b, and c are equal' : 'a, b, and c are not equal');//=> 'a, b, and c are not equal'

In this example, the ternary operator checks if a is equal to b and b is equal to c using the strict equality operator; much as we did above. If the comparison returns true, the output will be "a, b, and c are equal" (the lefthand side), otherwise, the output will be "a, b, and c are not equal" (the right‑hand side).

Using `Array.prototype.every`

A final option that I've used a few times ‑ and is very useful if you've got a lot of values to compare ‑ is the Array.prototype.every method, which we can use to check if all elements in an array are equal:

let a = 1;let b = '1';let c = 1;let arr = [a, b, c];console.log(arr.every((val) => val === a));//=> false

In this example, we use the Array.prototype.every method to check if each element in the array is equal to the first (a). Again ‑ using the strict equality operator ‑ means that we will get back false on this one due to the type mismatch between the variables.