Add linearSearch

snippets/linearSearch.md

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+---
+title: linearSearch
+tags: algorithm,array,beginner
+---
+
+Finds the first index of a given element in an array using the linear search algorithm.
+
+- Use a `for...in` loop to iterate over the indexes of the given array.
+- Check if the element in the corresponding index is equal to `item`.
+- If the element is found, return the index, using the unary `+` operator to convert it from a string to a number.
+- If the element is not found after iterating over the whole array, return `-1`.
+
+```js
+const linearSearch = (arr, item) => {
+  for (const i in arr) {
+    if (arr[i] === item) return +i;
+  }
+  return -1;
+};
+```
+
+```js
+linearSearch([2, 9, 9], 9); // 1
+linearSearch([2, 9, 9], 7); // -1
+```