feat(transmute): infer transmuter extra param based on transmute call

- Add conditional type inference that matches transmute usage
- When extra is provided to transmute, it's non-optional in transmuters
- When extra is omitted from transmute, transmuter can't access extra
- Improve readability with clearer variable names and logical grouping

The extra parameter in transmuters now matches how transmute is called:
present and required when extra is provided, absent when it's not.

src/__tests__/index.test.ts

@@ -70,7 +70,7 @@ describe('transmute', () => {
       {
         to: 'location',
         from: ({ source, extra }) =>
-          `${source.address.city}, ${source.address.country}${extra?.separator}`
+          `${source.address.city}, ${source.address.country}${extra.separator}`
       }
     ]
 

src/index.ts

@@ -12,15 +12,23 @@ export * from './types'
  * @param extra - Optional extra data to pass to mutation functions
  * @returns Transmuted object matching Target type
  */
-export const transmute = <Source, Target, Extra = unknown>(
+export const transmute = <Source, Target, Extra>(
   schema: Schema<Source, Target, Extra>[],
   source: Source,
   extra?: Extra
-): Target =>
-  schema.reduce<Target>(
-    (acc, { from, to }) => ({
-      ...acc,
-      [to]: typeof from === 'function' ? from({ source, extra }) : source[from]
-    }),
+): Target => {
+  return schema.reduce<Target>(
+    (acc, { from, to }) => {
+
+      const isFunction = typeof from === 'function'
+      const extraData = extra ? { extra } : {}
+      const value = isFunction ? (from as Function)({ source, ...extraData }) : source[from]
+
+      return {
+        ...acc,
+        [to]: value
+      }
+    },
     {} as Target
   )
+}

src/types.ts

@@ -3,12 +3,9 @@
  * @template Source - The source type being transmuted from
  * @template Extra - Type of additional data for transmutation
  */
-export type TransmuteFnArgs<Source, Extra> = {
-  /** The source object being transmuted */
-  source: Source
-  /** Optional extra data to assist with transmutation */
-  extra?: Extra
-}
+export type TransmuteFnArgs<Source, Extra> = Extra extends undefined | never
+  ? { source: Source }
+  : { source: Source; extra: Extra }
 
 /**
  * Function that performs a custom transmutation on a source object
@@ -33,11 +30,9 @@ type AssignableKeys<Source, Target, TargetKey extends keyof Target> = {
  * @template Target - The target type being transmuted to
  * @template Extra - Type of additional data for transmutation
  */
-export type Schema<Source, Target, Extra = unknown> = {
+export type Schema<Source, Target, Extra = undefined> = {
   [TargetKey in keyof Target]: {
-    /** Target property key */
     to: TargetKey
-    /** Source property key for direct mapping or a custom transmutation function */
     from: AssignableKeys<Source, Target, TargetKey> | TransmuteFn<Source, Target, TargetKey, Extra>
   }
 }[keyof Target]