From abca13e2603e2ada36481da21ecae6df8b470a68 Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonyg@leastfixedpoint.com>
Date: Sun, 17 Jan 2021 13:37:16 +0100
Subject: [PATCH] Idempotent Map/Set extensions

---
 implementations/javascript/src/flex.ts | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/implementations/javascript/src/flex.ts b/implementations/javascript/src/flex.ts
index 7cc4585..687a167 100644
--- a/implementations/javascript/src/flex.ts
+++ b/implementations/javascript/src/flex.ts
@@ -31,10 +31,14 @@ declare global {
     interface Set<T> { [IsSet]: boolean; }
     interface SetConstructor { isSet<T>(x: any): x is Set<T>; }
 }
-Object.defineProperty(Map.prototype, IsMap, { get() { return true; } });
-Map.isMap = <K,V> (x: any): x is Map<K, V> => !!x?.[IsMap];
-Object.defineProperty(Set.prototype, IsSet, { get() { return true; } });
-Set.isSet = <T> (x: any): x is Set<T> => !!x?.[IsSet];
+if (!(IsMap in Map.prototype)) {
+    Object.defineProperty(Map.prototype, IsMap, { get() { return true; } });
+    Map.isMap = <K,V> (x: any): x is Map<K, V> => !!x?.[IsMap];
+}
+if (!(IsSet in Set.prototype)) {
+    Object.defineProperty(Set.prototype, IsSet, { get() { return true; } });
+    Set.isSet = <T> (x: any): x is Set<T> => !!x?.[IsSet];
+}
 
 export function _iterMap<S,T>(i: Iterator<S>, f : (s: S) => T): IterableIterator<T> {
     const _f = (r: IteratorResult<S>): IteratorResult<T> => {