1 files changed, 14 insertions, 0 deletions
diff --git a/js/src/Y.js b/js/src/Y.js
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index 000000000..98d3257cb
--- /dev/null
+++ b/js/src/Y.js
@@ -0,0 +1,14 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// The Y combinator, applied to the factorial function.
+
+// Return the function that is the fixed point of f.
+const Y = f => (x => f(v => x(x)(v)))
+               (x => f(v => x(x)(v)));
+
+// The factorial function is the fixed point of this:
+const f = fac => n => (n <= 1) ? 1 : n * fac(n - 1);
+
+print(`5! is ${Y(f)(5)}`);