\
Sometimes, with arrays, we want to test every element for a certain condition. While individual conditions can be tested easily with an `if` statement, it becomes trickier with multiple array elements. As such, Javascript provides a method on arrays called `every` which will test every element for a specific test. If all pass the test, the overall `every` method will pass `true`. If one or more fail the test, then the overall `every` method will return `false`.

## Checking every element of an array for a test

Suppose we have an array where we want to test if every number is above 15. This is a perfect place to use `every`. Let's take a look.

```javascript
let arr = [ 25, 35, 45, 55, 65 ];
let check = arr.every((el, index, array) => {
    return el > 15
});

console.log(check); // true
```

\
`every` takes a function like `(el, index, array) => ...`.

* `el` is the current element being iterated through by `every`.
* `index` is the index of the current element being iterated through by the array.
* `array` is the full array being iterated upon. Here it would be `[ 25, 35, 45, 55, 65 ]`.

\
Since arrow functions will return true if on one line without a `return` statement, the above code can be simplified to this, too:

```javascript

let arr = [ 25, 35, 45, 55, 65 ];
let check = arr.every((el, index, array) => el > 15);

console.log(check); // true
```

\
Furthermore, the `every` method can take another form where it accepts a callback function's name, along with a custom value for `this`. That means you can create a function, and then prep its content based on a custom `this` within the function itself. For example, below I pass `{value: 15}` into `this`, allowing us to change this value should we want to for other times when we call `every`:

```javascript
let callbackFn = function(el) {
    if(this.value !== undefined) {
        return el > this.value
    }
    return false;
}

let arr = [ 25, 35, 45, 55, 65 ];
let check = arr.every(callbackFn, { value: 15 }); // Returns true
```

## Modifying an array with the every method

Much like other array methods, Javascript does allow modification of the array within a method called upon it. For example, we could modify each element of our array and then do a check on it. 

\
Since `every` is essentially a type of loop, each element is looped over sequentially:

```javascript
let arr = [ 25, 35, 45, 55, 65 ];
let check = arr.every((el, index, array) => {
    arr[index] -= 100;
    return el > 5
});

// This is true, since we subtracted 100 from each element before the check
console.log(check); // true
```

\
We can even delete elements from an array in the `every` loop:

```javascript
let arr = [ 25, 35, 45, 55, 65 ];
let check = arr.every((el, index, array) => {
    arr.pop();
    return el > 15;
});

console.log(arr); // returns [ 25, 35 ]
```

\
Javascript even lets you add elements to an array in `every`. Since `every` fires only once for each element in an array, it cannot lead to an infinite loop by simply pushing elements to the original array:

```javascript
let arr = [ 25, 35, 45, 55, 65 ];
let check = arr.every((el, index, array) => {
    arr.push(100);
    return el > 15;
});

// This is true, since we subtracted 100 from each element before the check
console.log(arr); // returns [25, 35, 45, 55, 65, 100, 100, 100, 100, 100]
console.log(check); // returns true
```

## Conclusion

The `every` method is a built-in way to carry out logical tests on entire arrays. You should be careful with this on large arrays as it may lead to performance problems. In most cases, though, it's a really useful tool to add to your array arsenal. There are many other use cases where `every` is very useful. For example, you can also use `every`  to [check if an array is a subset of another array](https://hackernoon.com/determine-whether-an-array-in-javascript-is-a-subset-of-another-array).

\

---

Also published [here.](https://fjolt.com/article/javascript-array-every) 

Too Long; Didn't Read

Cassandra Forward a free Cassandra-focused community event

The Array Every Method in JavaScript

Too Long; Didn't Read

@smpnjn

RELATED STORIES