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1-js/06-advanced-functions/01-recursion/02-factorial/solution.md

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+By definition, a factorial is `n!` can be written as `n * (n-1)!`.
+
+In other words, the result of `factorial(n)` can be calculated as `n` multiplied by the result of `factorial(n-1)`. And the call for `n-1` can recursively descend lower, and lower, till `1`.
+
+```js run
+function factorial(n) {
+  return (n != 1) ? n * factorial(n - 1) : 1;
+}
+
+alert( factorial(5) ); // 120
+```
+
+The basis of recursion is the value `1`. We can also make `0` the basis here, doesn't matter much, but gives one more recursive step:
+
+```js run
+function factorial(n) {
+  return n ? n * factorial(n - 1) : 1;
+}
+
+alert( factorial(5) ); // 120
+```

1-js/06-advanced-functions/01-recursion/02-factorial/task.md

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+importance: 4
+
+---
+
+# Calculate factorial
+
+The [factorial](https://en.wikipedia.org/wiki/Factorial) of a natural number is a number multiplied by `"number minus one"`, then by `"number minus two"`, and so on till `1`. The factorial of `n` is denoted as `n!`
+
+We can write a definition of factorial like this:
+
+```js
+n! = n * (n - 1) * (n - 2) * ...*1
+```
+
+Values of factorials for different `n`:
+
+```js
+1! = 1
+2! = 2 * 1 = 2
+3! = 3 * 2 * 1 = 6
+4! = 4 * 3 * 2 * 1 = 24
+5! = 5 * 4 * 3 * 2 * 1 = 120
+```
+
+The task is to write a function `factorial(n)` that calculates `n!` using recursive calls.
+
+```js
+alert( factorial(5) ); // 120
+```
+
+P.S. Hint: `n!` can be written as `n * (n-1)!` For instance: `3! = 3*2! = 3*2*1! = 6`