Array.prototype.every()

Summary: in this tutorial, you will learn how to use the JavaScript Array every() method to check whether all elements in an array pass a test.

Introduction to JavaScript Array every() method

The every() method that allows you to check if every element of an array passes a test provided by a function.

Here’s the syntax of the every() method:

array.every(callbackFn, thisArg)Code language: JavaScript (javascript)

The every() method accepts two named arguments: callbackFn and thisArg.

The callbackFn is a function that tests each element of the array. It has has the following form:

function callback(currentElement, index, array){
   //...
}Code language: JavaScript (javascript)

The callback() function takes three arguments:

• currentElement is the current element in the array being processed.
• index is the index  of the currentElement.
• array is the array that calls the every() method.

The currentElement is required whereas the index and array are optional.

The callbackFn returns a truthy value indicating that the element passes the test, and a falsy value otherwise.

The thisArg is an optional argument that can used as this value inside the callbackFn.

The every() method executes the callback() function on every element in the array until it finds the one that causes the callback() return a falsy value.

In other words, the every() will stop calling the callback() function and return false if it encounters an element that causes callback() to return a falsy value.

JavaScript Array every method examples

Let’s take some examples of using the every() method.

Basic Array every() method examples

The following example uses the every() to check if every element of the numbers array is greater than zero:

const numbers = [1, 3, 5];
const result = numbers.every((n) => n > 0);

console.log({ result });Code language: JavaScript (javascript)

Output:

{ result: true }Code language: JavaScript (javascript)

Since all the numbers in the numbers array are greater than zero, the every() method returns true.

The following example uses the every() method to test if all the numbers in the numbers array elements are even:

let numbers = [2, 4, 5];
let result = numbers.every((n) => n % 2 == 0);

console.log({ result });Code language: JavaScript (javascript)

Output:

{ result: false }Code language: JavaScript (javascript)

The numbers array has an odd number (5), so the every() method returns false.

Using the thisArg argument

The following example tests whether all elements in the numbers array is in the range specified by the min and max of the range object.

const numbers = [2, 4, 7];

const range = {
  min: 0,
  max: 10,
};

const isInRange = (n) => n >= this.min && n <= this.max;

const result = numbers.every(isInRange, range);

console.log({ result });Code language: JavaScript (javascript)

Output:

{ result: true }Code language: JavaScript (javascript)

How it works.

First, define an array of numbers:

const numbers = [2, 4, 7];Code language: JavaScript (javascript)

Next, define an object range that has two properties min and max:

const range = {
  min: 0,
  max: 10,
};Code language: JavaScript (javascript)

Then, define the isInRange method that checks if a number is in the range of min or max:

const isInRange = (n) => n >= this.min && n <= this.max;Code language: JavaScript (javascript)

After that, execute the isInRange method on every element of the numbers array using the every() method:

const result = numbers.every(isInRange, range);Code language: JavaScript (javascript)

We pass the range object to the every() method as the second argument. Inside the isInRage() function, we reference the range object using the this keyword.

Finally, display the result to the console:

console.log({ result });Code language: JavaScript (javascript)

Working with empty arrays

If you call the every() method on an empty array, the method will always return true for any condition. For example:

const gtZero = [].every((n) => n > 0);
console.log({ gtZero });

const ltZero = [].every((n) => n < 0);
console.log({ ltZero });Code language: JavaScript (javascript)

Output:

gtZero: true
ltZero: trueCode language: JavaScript (javascript)

Summary

• Use the Array every() method to test whether all elements in an array pass the test provided by a function.