Check if Not in Array in JavaScript

To check if an element is not included in an array in JavaScript, you can use the Array.prototype.includes() method along with the ! operator. This method returns a Boolean value indicating whether the specified element is found in the array. Here is an example:

const numbers = [1, 2, 3, 4, 5];

if (!numbers.includes(6)) {
  console.log('6 is not in the array');
}
Code language: JavaScript (javascript)

In this code, we first define an array called numbers that contains the numbers 1 through 5. Then, we use the Array.prototype.includes() method along with the ! operator to check if 6 is not in the numbers array. Since is not found, the code will print a message to the console.

Using Array.prototype.find()

If you have some more advanced logic required then you can use Array.prototype.find because you can pass in a function for determining if a value matches your given criteria, rather than just checking if a value doesn’t exist.

For example:

// Define an array of objects representing a list of employees
const employees = [
  { id: 1, name: "Alice", department: "Sales" },
  { id: 2, name: "Bob", department: "Marketing" },
  { id: 3, name: "Carol", department: "Sales" }
];

// Check if the employee with the specified ID is not in the array of employees
if (!employees.find(employee => {
  return employee.id === 4 && employee.department === "Sales";
})) {
  console.log(`Employee with ID: 4 and Department: Sales was not found`);
}
Code language: JavaScript (javascript)

The callback function for the find() method checks if the employee has the specified ID and is in the “Sales” department. If the find() method returns undefined, it means that the employee was not found in the array, and the code will print a message to the console.

Using indexOf()

Another, sort of outdated, way to check if a value is not in an array in JavaScript is to use the Array.prototype.indexOf() method. This method returns the index of the specified element in the array, or -1 if the element is not found. Here is an example:

const numbers = [1, 2, 3, 4, 5];

if (numbers.indexOf(6) === -1) {
  console.log('6 is not in the array');
}
Code language: JavaScript (javascript)

To check if a value is in an array using indexOf then you would check that the value returned by indexOf is greater than -1:

const numbers = [1, 2, 3, 4, 5];

if (numbers.indexOf(5) > -1) {
  console.log('5 is in the array');
}
Code language: JavaScript (javascript)

Or you can check for greater or equal to 0. But make sure to use >= and not >!

if (numbers.indexOf(1) >= 0) {
  console.log('1 is in the array');
}
Code language: JavaScript (javascript)

What Not to Do

These methods also work, but shouldn’t be used because the above methods are more concise and performant.

Using Array.protoype.every()

The Array.prototype.every() method returns a boolean indicating whether all elements in the array satisfy a given condition.

if (!myArray.every(val => val !== myValue)) {
    // myValue is not in myArray
}
Code language: JavaScript (javascript)

Using Array.protoype.filter()

Array.prototype.filter creates a new array with all elements that pass a given condition. If the resulting array is empty, it means that the value is not present in the original array.

if (myArray.filter(val => val === myValue).length === 0) {
    // myValue is not in myArray
}
Code language: JavaScript (javascript)