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A Definitive guide on JavaScript every Method
13 min read

A Definitive guide on JavaScript every Method

This article was last updated on February 7, 2024, to reorganize content for clarity and add new 'use cases' sections for the JavaScript 'every' method.

Introduction

This article is about the Array every() method in JavaScript. This is the third part of the JavaScript Iteration Methods Series.

Refinedev JavaScript Iteration Methods Series is a tutorial series that publishes posts on iteration methods in JavaScript.

The JavaScript Array every() method is an iteration method that tests whether every element in a collection satisfies a given condition passed via a callback function. It is used to verify if all items in an array fulfill some requirement, typically expressed in the form of a complex set of conditions.

In this post, we expound with examples what Array.prototype.every is and how it works. We get to know what entails the callback function that is used to set the test logic and cover examples of using it with and without the thisArg. We also examine the impact of using arrow syntax for the callback on the thisArg object. Later on, we investigate how to modify the caller array from inside with JavaScript every(). In the end, we explore how the JS every() method works with sparse and empty arrays.

Steps we'll cover:

What is the JavaScript Array.prototype.every Method ?

Array.prototype.every is a JavaScript iteration method that checks whether every element in an array satisfies a given condition. The method is called on an array of items, and the condition is checked with a callback function, callbackFn, and any necessary thisArg object passed to the execution context of the callback function.

The first argument, callbackFn, is mandatory, and the second argument, thisArg is optional. So, possible call signatures of the JavaScript every() method are:

// Method signature

every(callbackFn);
every(callbackFn, thisArg);

JS Array.prototype.every: Details of the Callback Function

The callbackFn() of every takes three arguments. The first is the element being traversed to, element, which is mandatory. The second argument is the current index, index and the third is array, the array being iterated. Both the second and third arguments are optional:

// Method signature

every(function(element){...});
every(function(element, index){...});
every(function(element, index, array){...});

How JavaScript Array every Method Works

JavaScript every traverses till the end of the array to test whether all elements satisfy the condition specified in the callback function,callbackFn. It attempts to execute callbackFn once for each item in the array. If it finds one that evaluates to a falsy value, it immediately returns with the Boolean false. Otherwise, it seeks to traverse to the end of the array and returns true if all are truthy:

const numbers = [1, 2, 3, 4, 5];
const numbersDoubled = [2, 4, 6, 8, 10];

const even = (element) => element % 2 === 0;

const areAllEven = numbers.every(even);
const areAllDoubledEven = numbersDoubled.every(even);

console.log(areAllEven); // false
console.log(areAllDoubledEven); // true

In the chunk of code above, even is our callback function, which we pass to every(). Apparently, we have at least one odd number in our numbers array and so not all elements pass the test. So, every() returns false for areAllEven. In contrast, all items in numbersDoubled are even, so we get true for areAllDoubledEven.

When to Use the JS Array every Method

The JS Array every method can be used in a wide range of scenarios where we need to verify that all items, with no exception, satisfy the test specified in the callback function. In all cases, the first argument (element) has to be passed to the callback. The complexity of the test would determine whether to access the second (index), third arguments (array), or any additional argument via the thisArg object.

We can use JS every() in the following situations:

  1. Simple linear conditionals: we can use JS every() for testing elements with simple linear conditionals like the even function above.
  2. Multiple linear conditionals: JS Array every() is more frequently used with complex tests that use multiple conditionals involving the element and the index.
  3. Complex conditionals with additional arguments: it is typical to use Array.prototype.every with additional arguments passed to its execution context via thisArg properties.
  4. Conditionals involving nested objects: it is also common to use Array every() with arrays of deeply nested objects.

JavaScript Array every() With thisArg Argument

We can pass in the thisArg object to every() and add it to the execution context of the callback function. Let's start doing that now by first making some modifications to our callback.

Instead of checking for an even number, let's say we want to generalize our callback function to check if the item is divisible by a given number. We would like our callback to be something like the below:

function divisible(element, divisor) {
return element % divisor === 0;
}

However, we cannot pass divisor as the second argument to divisible(), as our callback accepts index and array as the second and third arguments respectively. And it becomes overcrowded if we introduce a fourth with divisor.

We can get around this problem by passing divisor as a property of the thisArg object, the second argument to JavaScript every(). And then access the object with this from inside the callback:

const numbers = [1, 2, 3, 4, 5];
const numbersDoubled = [2, 4, 6, 8, 10];

function divisible(element) {
return element % this?.divisor === 0;
}

const areAllEven = numbers.every(divisible, { divisor: 2 });
const areAllDoubledEven = numbersDoubled.every(divisible, { divisor: 2 });

console.log(areAllEven); // false
console.log(areAllDoubledEven); // true

Here, we set the thisArg object to { divisor: 2 }, which leads to checking whether the item is even or not.

We can try other divisor options, like checking if we have a number divisible by 3 or 7. Thanks to thisArg, this became very easily reproducible:

const areAllDivisibleByThree = numbers.every(divisible, { divisor: 3 });
const areAllDivisibleBySeven = numbers.every(divisible, { divisor: 7 });

console.log(areAllDivisibleByThree); // false
console.log(areAllDivisibleBySeven); // false

JavaScript Array every(callback, thisArg) Doesn't Work With Arrow Functions

If we look back at the first example that involves the even() callback, we defined it as an arrow function. And it worked.

We defined its extension, the divisible() function with named declaration syntax. And it worked as well.

If we declare divisible() as an arrow function, we run into problems:

const numbers = [1, 2, 3, 4, 5];
const numbersDoubled = [2, 4, 6, 8, 10];

const divisible = (element) => element % this?.divisor === 0;

const areAllDoubledEven = numbersDoubled.every(divisible, { divisor: 2 });
const areAllDoubledDivisibleByThree = numbersDoubled.every(divisible, {
divisor: 3,
});
const areAllDoubledDivisibleBySeven = numbersDoubled.every(divisible, {
divisor: 7,
});

console.log(areAllDoubledEven); // false
console.log(areAllDoubledDivisibleByThree); // false
console.log(areAllDoubledDivisibleBySeven); // false

All returning false, although we know areAllDoubledEven should be true and the other two false.

If we investigate with a modified divisible() function that logs this to the console, we see that this is undefined in strict mode:

// strict mode

const numbers = [1, 2, 3, 4, 5];
const numbersDoubled = [2, 4, 6, 8, 10];

const divisible = (element) => {
console.log(this);
return element % this?.divisor === 0;
};

const areAllDoubledEven = numbersDoubled.every(divisible, { divisor: 2 });

console.log(areAllDoubledEven);
// undefined
// false

Now, if we introduce a this.divisor property to the lexical environment of divisible(), we get its value logged to the console:

const numbers = [1, 2, 3, 4, 5];
const numbersDoubled = [2, 4, 6, 8, 10];

this.divisor = "Hi";

const divisible = (element) => {
console.log(this);
return element % this.divisor === 0;
};

const areAllDoubledEven = numbersDoubled.every(divisible, { divisor: 2 });

console.log(areAllDoubledEven);
// { divisor: 'Hi' }
// false

Here, clearly, we have { divisor: 'Hi' } coming from divisible's closure. It turns out, the problem is due to the binding of divisible()'s this to its lexical environment because of the arrow syntax. It was undefined before we introduced this.divisor = 'Hi';. Now this is { divisor: 'Hi' }. In other words, { divisor: 2 } is not being relayed to divisible's this.

So, JavaScript every() with thisArg does not work as expected with callbackFn defined with arrow syntax.

JavaScript Array every(callback, thisArg) Works With Non-Arrow Functions

But as we have seen before, it works with callbacks defined with named function declarations:

function divisible(element) {
return element % this?.divisor === 0;
}

const areAllDoubledEven = numbersDoubled.every(divisible, { divisor: 2 });

console.log(areAllDoubledEven); // true

It also works with anonymous function expressions:

const divisible = function (element) {
return element % this?.divisor === 0;
};

const areAllDoubledEven = numbersDoubled.every(divisible, { divisor: 2 });

console.log(areAllDoubledEven); // true

JavaScript Array every Method: Modifying the Caller Array

Array.prototype.every sets the range of the items to be processed before the first invocation of the callback function. And if an item is changed after traversal, the change is disregarded by the callback function. That is, the callback function only respects the existing value of an item at the time it is visited.

We can witness this in a scenario where the caller array is mutated from inside JavaScript every().

every() itself does not modify the caller array, but the caller is available to the callback function as its third argument, array. This means we can deliberately mutate the caller when we need to from inside our callback, divisible():

function divisible(element, index, array) {
array[0] = 7;
array[4] = 7;

console.log(array);

return element % this?.divisor === 0;
}

In this scenario, if an unvisited item is changed ahead of time, the callback function - here divisible - finds the new value when it visits the item and so the new value is processed. In contrast, it disregards changes to items that are already traversed:

const numbers = [1, 2, 3, 4, 5];
const numbersDoubled = [2, 4, 6, 8, 10];

const divisible = function (element, index, array) {
array[0] = 7;
array[4] = 7;

console.log(array);

return element % this?.divisor === 0;
};

const areAllDoubledEven = numbersDoubled.every(divisible, { divisor: 2 });
console.log(areAllDoubledEven);
console.log(numbersDoubled);
/*
[ 7, 4, 6, 8, 7 ]
[ 7, 4, 6, 8, 7 ]
[ 7, 4, 6, 8, 7 ]
[ 7, 4, 6, 8, 7 ]
[ 7, 4, 6, 8, 7 ]
false
[ 7, 4, 6, 8, 7 ]
*/

In the console log statements above, the numbersDoubled array is being logged five times due to the console.log(array); statement we placed inside divisible().

As we can see, numbersDoubled is being mutated twice in the first call to divisible(). The first mutation happens when at numbersDoubled[0], i.e. after being visited, which changes the value of itself to 7. So, even though 7 is not divisible by the divisor 2, every() didn't immediately return false at index 0. Instead, it returned false in the next instance when it visited the unvisited and mutated value of 7 at numbersDoubled[4].

This shows that the callback function processes the value of an item as it finds it at traversal and disregards the changes made to it when and after it is at that index.

Nuances of Using JS Array every Method

The JS Array.prototype.every: Using with Sparse Arrays

Now, let's consider what happens when we have empty slots in the caller array.

We'll add a couple of empty items to numbersDouble and remove the mutations from divisible:

const numbers = [1, 2, 3, 4, 5];
const numbersDoubled = [2, 4, , 6, 8, , 10];

const divisible = function (element, index, array) {
console.log(array);
return element % this?.divisor === 0;
};

const areAllDoubledEven = numbersDoubled.every(divisible, { divisor: 2 });
console.log(areAllDoubledEven);
console.log("Caller length: " + numbersDoubled.length);
/*
[ 2, 4, <1 empty item>, 6, 8, <1 empty item>, 10 ]
[ 2, 4, <1 empty item>, 6, 8, <1 empty item>, 10 ]
[ 2, 4, <1 empty item>, 6, 8, <1 empty item>, 10 ]
[ 2, 4, <1 empty item>, 6, 8, <1 empty item>, 10 ]
[ 2, 4, <1 empty item>, 6, 8, <1 empty item>, 10 ]
true
[ 2, 4, <1 empty item>, 6, 8, <1 empty item>, 10 ]
*/

As we can see, we have two empty slots and the length of the caller array is 7. However, the numbersDoubled is logged 5 times, indicating 5 calls to divisible(). This is because divisible() was not invoked for the empty items.

Using JavaScript every With Empty Arrays

If we call every() with divisible on an empty array, it returns true:

const emptyArray = [];

const divisible = (element) => {
return element % this?.divisor === 0;
};

const testEmptyArray = emptyArray.every(divisible, { divisor: 2 });

console.log(testEmptyArray); // true

This is so because "all items" in an empty array vacuously satisfy the condition that they are even or anything else. Supposedly.

Difference Between Some and Every in JavaScript?

In JavaScript, some() returns true if any array element meets a condition, while every() checks if all elements do. Neither modifies the original array.

  • JavaScript some():

  • Checks if at least one element in an array meets a condition.

  • Returns true if any element passes the test, otherwise false.

  • Stops checking once it finds a passing element.

  • JavaScript every():

    • Checks if every element in an array meets a condition.
    • Returns true only if all elements pass the test, otherwise false.
    • Stops checking once it finds a failing element.

Summary

In this article, we explored in depth the JavaScript every which helps us test whether all items in an array pass the test we implement in a callback function. We saw with examples that the callback function can take three arguments, and additional arguments can be bound to its execution context by setting its this value with a thisArg passed to every() as the second argument.

We also found out that if we use arrow syntax to declare the callback function, its lexical environment disrupts the binding of thisArg argument to its this object. So, we should be using non-arrow functions to declare a callback that uses this.

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