feat: Explain the behavior of const keyword in JavaScript

Questions-Answers/76.const.md

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+# 76. const
+
+### Problem
+
+https://bigfrontend.dev/quiz/const
+
+#
+
+### Problem Description
+
+What does the code snippet below output by `console.log`?
+
+<!-- prettier-ignore -->
+```js
+function func() {
+  const a = b = c = 1;
+}
+func();
+console.log(typeof a, typeof b, typeof c);
+```
+
+#
+
+### Answer
+
+```
+"undefined","number","number"
+```
+
+### Explanation
+
+- `typeof a` returns `"undefined"`, because constants are block-scoped. We declared `a` with `const` inside the function `func()`, therefore `a` is local only to the `func() {}` block. When we access `a` outside of that scope, i.e., in the global scope, `a` is `undefined`.
+
+- `typeof b` and `typeof c` both return `"number"`, because `b` and `c` are declared globally.
+
+  `const a = b = c = 1` is equivalent to the following:
+
+  ```js
+  c = 1;
+  b = c;
+
+  const a = b;
+  ```
+
+  Since there are no declarations for `b` and `c`, both of them are created as global variables. Therefore, the code snippet in the problem statement is essentially the same as the following:
+
+  ```js
+  var b;
+  var c;
+
+  function func() {
+    c = 1;
+    b = c;
+
+    const a = b;
+  }
+  func();
+  console.log(typeof a, typeof b, typeof c);
+  ```
+
+  First, we call `func()`; `c` and `b` are assigned to `1`. Then we log the type of `b` and `c`. Since now `b` and `c` are pointing to the number value `1`, `typeof b` and `typeof c` return `"number"`.
+
+  It's worth noting, when declaring a constant, we must specify its value in the same statement in which it is declared:
+
+  ```js
+  // throws an error
+  // Uncaught SyntaxError: Missing initializer in const declaration
+
+  const a;
+  ```
+
+#
+
+### Reference
+
+[Initialization of several variables](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var#initialization_of_several_variables)