Update js-array-methods.md

cheatsnake · cheatsnake · commit 3522d2833b92 · 2021-12-28T10:19:46.000+03:00
diff --git a/JavaScript/Array methods/js-array-methods.md b/JavaScript/Array methods/js-array-methods.md
@@ -65,3 +65,113 @@ const arr = [1, 2, 3, 4, 5];
 const newArr = arr.slice(1, 4);
 // newArr = [2, 3, 4]
 ```
+
+## Search among elements
+
+-   indexOf/lastIndexOf(item, pos) <br>
+    look for `item` starting from index `pos`, return the index or -1 if not found. Method `lastIndexOf()` searches from right to left
+
+```js
+const arr = [10, 20, 20, 30];
+const first = arr.indexOf(20); // first = 1
+const last = arr.lastIndexOf(20); // last = 2
+```
+
+-   includes(value) <br>
+    returns true if the array has value, else false
+
+```js
+const arr = [10, 20, 30, 40];
+const n = arr.includes(15); // false
+```
+
+-   find/findIndex(func(item, index, arr)) <br>
+    `func()` is called for each element of the array. If the `func()` returns true, the search is interrupted and item is returned, else - undefined
+
+```js
+const arr = [1, 13, 23, 43, 56, 71, 97]
+const evenNum = arr.find(item => {
+    return % 2 === 0
+})
+// evenNum = 56
+```
+
+-   filter(func(item, index, arr)) <br>
+    the method is similar to `find()`, but `filter()` returns an array of all matching elements
+
+```js
+const arr = [1, 2, 3, 4, 5, 6, 7];
+const evenArr = arr.filter((item) => {
+    return item % 2 === 0;
+}); // evenArr = [2, 4, 6]
+```
+
+## Iteration of elements
+
+-   forEach(func(item, index, array)) <br>
+    calls `func()` for every element in array, does not return anything
+
+```js
+const arr = [1, 2, 3];
+arr.forEach((item, index, arr) => {
+    console.log(item, index, arr);
+});
+// 1 0 [1, 2, 3]
+// 2 1 [1, 2, 3]
+// 3 2 [1, 2, 3]
+```
+
+## Transform the array
+
+-   map(func(item, index, array)) <br>
+    calls the `func()` for each element of the array and returns the array of results
+
+```js
+const arr = [1, 2, 3, 4, 5];
+const newArr = arr.map((item) => {
+    return item * 10;
+});
+// newArr = [10, 20, 30, 40, 50]
+```
+
+-   sort(func()) <br>
+    sorts the array changing it's element order, `func()` defining the sorting order (optional).
+
+```js
+const arr = [25, 7, 13 31]
+arr.sort((a, b) => {
+    return a - b
+})
+// arr = [7, 13, 25, 31]
+```
+
+-   reverse() <br>
+    reverse the order of the elements of the current array
+
+```js
+const arr = [1, 2, 3, 4, 5];
+arr.reverse();
+// arr = [5, 4, 3, 2, 1]
+```
+
+-   join(separator)/string.split(separator) <br>
+    convert an array to string / a string to array
+
+```js
+const str = "one, two, three, four";
+const arr = str.split(", ");
+// arr = ["one", "two", "three", "four"]
+const newStr = arr.join("-");
+// newStr = "one-two-three-four"
+```
+
+-   reduce/reduceRight(func(accumulator, current, index, array), initial) <br>
+    calculate a single value over the array by calling `func()` for each element and passing an intermediate result between the calls
+
+```js
+const arr = [1, 2, 3, 4, 5];
+const sumFromElements = arr.reduce((sum, current) => {
+    return sum + current;
+}, 0);
+// sumFromElements = 15
+```
