closes #2136 #2150

Author: iliakan

1-js/05-data-types/04-array/article.md

@@ -429,6 +429,53 @@ alert( "1" + 1 ); // "11"
 alert( "1,2" + 1 ); // "1,21"
 ```
 
+## Dont compare arrays with ==
+
+Arrays in JavaScript, unlike some other programming languages, shouldn't be compared with operator `==`.
+
+This operator has no special treatment for arrays, it works with them as with any objects.
+
+Let's recall the rules:
+
+- Two objects are equal `==` only if they're references to the same object.
+- If one of arguments of `==` is an object, and the other one is a primitive, then the object gets converted to primitive, as explained in the chapter <info:object-toprimitive>.
+- ...With an exception of `null` and `undefined` that equal `==` each other and nothing else.
+
+The strict comparison `===` is even simpler, as it doesn't convert types. 
+
+So, if we compare arrays with `==`, they are never the same, unless we compare two variables that reference exactly the same array.
+
+For example:
+```js run
+alert( [] == [] ); // false
+alert( [0] == [0] ); // false
+```
+
+These arrays are technically different objects. So they aren't equal. The `==` operator doesn't do item-by-item comparison.
+
+Comparison with primitives may give seemingly strange results as well:
+
+```js run
+alert( 0 == [] ); // true
+
+alert('0' == [] ); // false
+```
+
+Here, in both cases, we compare a primitive with an array object. So the array `[]` gets converted to primitive and becomes an empty string `''`. 
+
+Then the comparison process goes on:
+
+```js run
+// [] converted to ''
+alert( 0 == '' ); // true, as '' becomes converted to number 0
+
+alert('0' == '' ); // false, no type conversion, different strings
+```
+
+So, how to compare arrays?
+
+That's simple: don't use the `==` operator. Instead, compare them item-by-item in a loop or using iteration methods explained in the next chapter.
+
 ## Summary
 
 Array is a special kind of object, suited to storing and managing ordered data items.
@@ -460,4 +507,8 @@ To loop over the elements of the array:
   - `for (let item of arr)` -- the modern syntax for items only,
   - `for (let i in arr)` -- never use.
 
-We will return to arrays and study more methods to add, remove, extract elements and sort arrays in the chapter <info:array-methods>.
+To compare arrays, don't use the `==` operator (as well as `>`, `<` and others), as they have no special treatment for arrays. They handle them as any objects, and it's not what we usually want.
+
+Instead you can use `for..of` loop to compare arrays item-by-item.
+
+We will continue with arrays and study more methods to add, remove, extract elements and sort arrays in the next chapter <info:array-methods>.

1-js/05-data-types/05-array-methods/article.md

@@ -742,6 +742,15 @@ These methods are the most used ones, they cover 99% of use cases. But there are
 
   The function `fn` is called on each element of the array similar to `map`. If any/all results are `true`, returns `true`, otherwise `false`.
 
+  We can use `every` to compare arrays:
+  ```js run
+  function arraysEqual(arr1, arr2) {
+    return arr1.length === arr2.length && arr1.every((value, index) => value === arr2[index]);
+  }
+
+  alert( arraysEqual([1, 2], [1, 2])); // true
+  ```
+
 - [arr.fill(value, start, end)](mdn:js/Array/fill) -- fills the array with repeating `value` from index `start` to `end`.
 
 - [arr.copyWithin(target, start, end)](mdn:js/Array/copyWithin) -- copies its elements from position `start` till position `end` into *itself*, at position `target` (overwrites existing).