Prevent ABI objects from being mutated

CHANGELOG.md

@@ -316,3 +316,4 @@ Released with 1.0.0-beta.37 code base.
 ### Changed
 
 - Remove `notImplemented` flag from ETH2 Beacon Chain package methods schema
+- Fixed mutation of inputs to encoding and decoding functions (#3748)

packages/web3-eth-abi/src/index.js

@@ -155,7 +155,7 @@ ABICoder.prototype.mapTypes = function (types) {
         // recognize former type. Solidity docs say `Function` is a bytes24
         // encoding the contract address followed by the function selector hash.
         if (typeof type === 'object' && type.type === 'function'){
-            type.type = "bytes24";
+            type = Object.assign({}, type, { type: "bytes24" });
         }
         if (self.isSimplifiedStructFormat(type)) {
             var structName = Object.keys(type)[0];

test/abi.decodeParameter.js

@@ -638,3 +638,18 @@ describe('lib/solidity/coder', function () {
                         '737472696e670000000000000000000000000000000000000000000000000000'})
     });
 });
+
+describe('/lib/solidity/coder', function() {
+    describe('decodeParam', function () {
+        it('should not alter inputs', function () {
+            const t = {
+                type: "function",
+                name: "f",
+                internalType: "function () external"
+            };
+            const copyOfT = Object.assign({}, t);
+            coder.decodeParameter(t, '063e4f349a9e91c6575aedab0e70087fab642ecac04062260000000000000000'); //must not alter t!
+            assert.deepEqual(t, copyOfT);
+        });
+    });
+});