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There are many cases where knowing when an array is a subset of another can be pretty useful - and although usually immediately obvious to the human eye, it can be difficult to evaluate in code.

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In the code below, `arr2` is a **subset** of `arr1`, but `arr3` is not.

```javascript
let arr1 = [ 'a', 'b', 'c' ];
let arr2 = [ 'b', 'c' ];
let arr3 = [ 'c', 'd' ];
```

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If we want to find out if an array is a subset of another array, the best way to evaluate this is by using the [array every method](https://hackernoon.com/the-array-every-method-in-javascript). This method iterates through each element in an array and performs a test on it. If every element in the array passes that test, then the overall `every` method will return true.

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For each evaluation, we have a "parent" array, and a "subset" array - where we want to check if the "subset" array is fully contained within the "parent" array.

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To evaluate if one array is a subset of another, we can run `every` on each element on the "subset" array. Within the `every` function, we can test if the "parent" array contains each element in the "subset" array. If it does, then `every` will return `true`. Otherwise, it'll return `false`.

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This can be accomplished easily with a function like the one shown below - `checkSubset` will return `true` should the `subsetArray` be fully contained within the `parentArray`:

```javascript
let arr1 = [ 'a', 'b', 'c' ];
let arr2 = [ 'b', 'c' ];
let arr3 = [ 'c', 'd' ];

let checkSubset = (parentArray, subsetArray) => {
    return subsetArray.every((el) => {
        return parentArray.includes(el)
    })
}

checkSubset(arr1, arr2); // returns true
checkSubset(arr1, arr3); // returns false
```

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Also published [here.](https://fjolt.com/article/javascript-check-if-array-is-subset)

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Determine Whether an Array in JavaScript Is a Subset of Another Array

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