Allow memoization of function parameter

src/create-transformer.ts

@@ -52,11 +52,12 @@ export function createTransformer<A, B>(
 }
 
 function getMemoizationId(object: any) {
-    if (typeof object === "string") return `string:${object}`
-    if (typeof object === "number") return `number:${object}`
-    if (object === null || typeof object !== "object")
+    const objectType = typeof object
+    if (objectType === "string") return `string:${object}`
+    if (objectType === "number") return `number:${object}`
+    if (object === null || (objectType !== "object" && objectType !== "function"))
         throw new Error(
-            "[mobx-utils] transform expected an object, string or number, got: " + object
+            `[mobx-utils] transform expected an object, function, string or number, got: ${String(object)}`
         )
     let tid = object.$transformId
     if (tid === undefined) {

test/__snapshots__/create-transformer.ts.snap

@@ -0,0 +1,9 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`should throw error when passed invalid param type 1`] = `"[mobx-utils] transform expected an object, function, string or number, got: null"`;
+
+exports[`should throw error when passed invalid param type 2`] = `"[mobx-utils] transform expected an object, function, string or number, got: undefined"`;
+
+exports[`should throw error when passed invalid param type 3`] = `"[mobx-utils] transform expected an object, function, string or number, got: Symbol(A)"`;
+
+exports[`should throw error when passed invalid param type 4`] = `"[mobx-utils] transform expected an object, function, string or number, got: true"`;

test/create-transformer.ts

@@ -1281,3 +1281,21 @@ function createTestSet(): any {
 
     return testSet
 }
+
+it("should throw error when passed invalid param type", () => {
+    const transformedFn = createTransformer((obj: any) => obj)
+    const validParams = [[], {}, jest.fn(), 1, "string"]
+    const invalidParams = [null, undefined, Symbol("A"), true]
+
+    validParams.forEach(obj => {
+        expect(() => {
+            transformedFn(obj)
+        }).not.toThrowError()
+    })
+
+    invalidParams.forEach(obj => {
+        expect(() => {
+            transformedFn(obj)
+        }).toThrowErrorMatchingSnapshot()
+    })
+})