Allow replacement to be a function

README.md

@@ -24,7 +24,7 @@ $ npm install mask-json
 Option        | Default value  | Description
 ------------- | -------------- | -----------------------------------------------------
 _ignoreCase_  | false          | Whether to ignore case sensitivity when matching keys
-_replacement_ | _--REDACTED--_ | The default value to replace
+_replacement_ | _--REDACTED--_ | The default value to replace, this can be either an explicit value or a function using current value as only parameter
 
 ### Returns
 
@@ -41,15 +41,31 @@ maskJson({ foo: 'bar', biz: { username: 'myusername', password: 'mypassword' } }
 // => { foo: 'bar', biz: { username: '--REDACTED--', password: '--REDACTED--' } }
 ```
 
-## Tests
+Works the same when "replacement" is not a static string, but a function to convert 
+the original value somehow, e.g. mask some parts of a credit card number or phone number
+like the original "1234-5678-9012" shall become "1234-****-9012".
+
+The replacement function is called with one parameter - the original value to replace.
 
 ```javascript
+// with replacement function
+var replaceFunc = function(value) { return value.toUpperCase() }; 
+var maskJson2 = require('mask-json')(blacklist, {replacement: replaceFunc});
+
+maskJson2({ foo: 'bar', biz: { username: 'myusername', password: 'mypassword' } });
+
+// => { foo: 'bar', biz: { username: 'MYUSERNAME', password: 'MYPASSWORD' } }
+```
+
+## Tests
+
+```shell script
 $ npm test
 ```
 
 ## Release
 
-```sh
+```shell script
 npm version [<new version> | major | minor | patch] -m "Release %s"
 ```
 

package.json

@@ -29,7 +29,7 @@
     "version": "npm run changelog && git add -A CHANGELOG.md"
   },
   "dependencies": {
-    "lodash": "^4.17.4"
+    "lodash": "^4.17.21"
   },
   "devDependencies": {
     "@uphold/github-changelog-generator": "^0.8.1",

src/index.js

@@ -17,7 +17,7 @@ module.exports = function maskJson(collection, {
     return cloneDeepWith(values, (value, key) => {
       // Strip matching keys.
       if (some(collection, item => ignoreCase ? toLower(key) === toLower(item) : key === item)) {
-        return replacement;
+        return (typeof replacement === 'function') ? replacement(value) : replacement;
       }
 
       // Allow cloneDeep to recurse into nested objects.

test/src/index.test.js

@@ -30,7 +30,7 @@ describe('maskJson()', () => {
     });
   });
 
-  it('should accept a custom `replacement`', () => {
+  it('should accept a custom `replacement` value', () => {
     const object = {
       foo: {
         password: 'foobar',
@@ -46,6 +46,26 @@ describe('maskJson()', () => {
     });
   });
 
+  it('should accept a custom `replacement` function', () => {
+    const object = {
+      foo: {
+        password: 'foobar',
+        secret: 'bizbaz'
+      }
+    };
+
+    function replacementFunc(value) {
+      return value.toUpperCase();
+    }
+
+    expect(maskJson(['password', 'secret'], { replacement: replacementFunc })(object)).toEqual({
+      foo: {
+        password: 'FOOBAR',
+        secret: 'BIZBAZ'
+      }
+    });
+  });
+
   it('should mask non-plain objects', () => {
     const object = {
       bar: {