Linear search, also called sequential or simple, is the most basic search algorithm. Given a data structure, for example an array, we search for an item by looking at all the elements, until we find it.

Its implementation is very simple:

```
const linearSearch = (list, item) => {
for (const [i, element] of list.entries()) {
if (element === item) {
return i
}
}
}
```

This returns the index of the item we’re looking for. Example:

`linearSearch(['a', 'b', 'c', 'd'], 'd') //3 (index start at 0)`

If we look for ‘a’, the algorithm will only look at the first element and return, so it’s very fast.

But if we look for the last element, the algorithm needs to loop through all the array. To calculate the Big O value we always look at the worst-case scenario.

So the algorithm complexity is `O(n)`

.