better type errors and a fix for #100

Author: dgreene1

README.md

@@ -8,15 +8,15 @@
 
 - [Design Goals](#design-goals)
 - [Why would you want to use this:](#why-would-you-want-to-use-this)
-  * [First, let's try to write the flexible factory function without mergePartially](#first-lets-try-to-write-the-flexible-factory-function-without-mergepartially)
-  * [Now let's refactor using mergePartially](#now-lets-refactor-using-mergepartially)
+  - [First, let's try to write the flexible factory function without mergePartially](#first-lets-try-to-write-the-flexible-factory-function-without-mergepartially)
+  - [Now let's refactor using mergePartially](#now-lets-refactor-using-mergepartially)
 - [Examples](#examples)
 - [F.A.Q. / Troubleshooting](#faq--troubleshooting)
-  * [Why wouldn't I just use Object.assign or the spread operator?](#why-wouldnt-i-just-use-objectassign-or-the-spread-operator)
-  * [I see lots of TypeScript stuff. Can I use this in JavaScript too?](#i-see-lots-of-typescript-stuff-can-i-use-this-in-javascript-too)
-  * [What's the difference between .deep and .shallow?](#whats-the-difference-between-deep-and-shallow)
-  * [Why is `.shallow` even necessary?](#why-is-shallow-even-necessary)
-  * [Why is my return type some strange error string?](#why-is-my-return-type-some-strange-error-string)
+  - [Why wouldn't I just use Object.assign or the spread operator?](#why-wouldnt-i-just-use-objectassign-or-the-spread-operator)
+  - [I see lots of TypeScript stuff. Can I use this in JavaScript too?](#i-see-lots-of-typescript-stuff-can-i-use-this-in-javascript-too)
+  - [What's the difference between .deep and .shallow?](#whats-the-difference-between-deep-and-shallow)
+  - [Why is `.shallow` even necessary?](#why-is-shallow-even-necessary)
+  - [Why is my return type some strange error string?](#why-is-my-return-type-some-strange-error-string)
 - [Contributions](#contributions)
 
 <!-- tocstop -->
@@ -157,6 +157,10 @@ const shallowResult = mergePartially.shallow(seed, {
 
 There are some data types that are "less-compatible" with the library and therefore require a workaround ([click here for the description](https://github.com/dgreene1/merge-partially/blob/master/whyShallowInstead.md)). It should be rare that you need to use `.shallow`, but you might prefer `.shallow` over `.deep` anyway for explicitness.
 
+### Why am I getting some error about `never`?
+
+If you're seeing a Typescript error similar to `"Type 'number' is not assignable to type 'never'"` then you it's likely a case where you need to inform the function what the type is for your `seed` parameter. See [this thread](https://github.com/dgreene1/merge-partially/issues/100#issuecomment-694866025) for a detailed answer and a fun description in how TypeScript works.
+
 ### Why is my return type some strange error string?
 
 In order to meet the design goals (see above), mergePartially proactively prevents certain data combinations. See this link for more information on the soluton: [https://github.com/dgreene1/merge-partially/blob/master/whyShallowInstead.md](https://github.com/dgreene1/merge-partially/blob/master/whyShallowInstead.md)

src/index.spec.ts

@@ -30,18 +30,23 @@ describe('mergePartially', () => {
     });
 
     it('should overwrite a boolean even a falsy (i.e. false)', () => {
-      const original = {
-        a: 'a',
-        b: true,
-      };
+      interface IHaveABoolAndStr {
+        a: string;
+        b: boolean;
+      }
 
-      const result = mergePartially.deep(original, {
-        b: false,
-      });
+      // Notice that.......... you can explicitly type the seed by passing in a <type> parameter
+      const result = mergePartially.deep<IHaveABoolAndStr>(
+        {
+          a: 'a',
+          b: true,
+        },
+        {
+          b: false,
+        }
+      );
 
       expect(result.b).toEqual(false);
-      // Prove that mergePartially is a pure function
-      expect(original.b).toEqual(true);
     });
 
     it('should not replace missing properties, but should replace present properties with falsy values', () => {

src/index.ts

@@ -122,9 +122,9 @@ const mergePartiallyShallow = <T1 extends object>(seed: T1, override: Partial<T1
  * @param seed the object that is used establish the start of what you want the result to look like. This is the object that will be overriden before a result is produced
  * @param override the data that will be used when replacing the seed's key/values
  */
-const mergePartiallyDeep = <T1 extends object, T2 extends NestedPartial<T1> | undefined = undefined>(
+const mergePartiallyDeep = <T1 extends object, T2 extends NestedPartial<T1> = NestedPartial<T1>>(
   seed: T1,
-  override: T2
+  override: T2 | undefined
 ): NoNestedOptionalObjectsDeep<T1> => {
   return mergePartiallyShallow(seed, override) as NoNestedOptionalObjectsDeep<T1>;
 };