Add a transform option (#45)

index.js

@@ -76,7 +76,11 @@ module.exports = (val, opts, pad) => {
 
 			const ret = '[' + tokens.newLine + val.map((el, i) => {
 				const eol = val.length - 1 === i ? tokens.newLine : ',' + tokens.newLineOrSpace;
-				return tokens.indent + stringify(el, opts, pad + opts.indent) + eol;
+				let value = stringify(el, opts, pad + opts.indent);
+				if (opts.transform) {
+					value = opts.transform(val, i, value);
+				}
+				return tokens.indent + value + eol;
 			}).join('') + tokens.pad + ']';
 
 			seen.pop(val);
@@ -102,7 +106,11 @@ module.exports = (val, opts, pad) => {
 				const isSymbol = typeof el === 'symbol';
 				const isClassic = !isSymbol && /^[a-z$_][a-z$_0-9]*$/i.test(el);
 				const key = isSymbol || isClassic ? el : stringify(el, opts);
-				return tokens.indent + String(key) + ': ' + stringify(val[el], opts, pad + opts.indent) + eol;
+				let value = stringify(val[el], opts, pad + opts.indent);
+				if (opts.transform) {
+					value = opts.transform(val, el, value);
+				}
+				return tokens.indent + String(key) + ': ' + value + eol;
 			}).join('') + tokens.pad + '}';
 
 			seen.pop(val);

readme.md

@@ -79,6 +79,41 @@ Type: `Function`
 
 Expected to return a `boolean` of whether to include the property `prop` of the object `obj` in the output.
 
+##### transform(obj, prop, originalResult)
+
+Type: `Function`<br>
+Default: `undefined`
+
+Expected to return a `string` that transforms the string that resulted from stringifying `obj[prop]`. This can be used to detect special types of objects that need to be stringified in a particular way. The `transform` function might return an alternate string in this case, otherwise returning the `originalResult`.
+
+Here's an example that uses the `transform` option to mask fields named "password":
+
+```js
+const obj = {
+	user: 'becky',
+	password: 'secret'
+}
+
+const pretty = stringifyObject(obj, {
+	transform: (obj, prop, originalResult) => {
+		if (prop === 'password') {
+			return originalResult.replace(/\w/g, '*');
+		} else {
+			return originalResult;
+		}
+	}
+});
+
+console.log(pretty);
+/*
+{
+	user: 'becky',
+	password: '******'
+}
+*/
+```
+
+
 ##### inlineCharacterLimit
 
 Type: `number`

test.js

@@ -72,6 +72,31 @@ it('considering filter option to stringify an object', () => {
 	assert.equal(actual, '{\n\tbar: {\n\t\tval: 10\n\t}\n}');
 });
 
+it('allows an object to be transformed', () => {
+	const obj = {
+		foo: {
+			val: 10
+		},
+		bar: 9,
+		baz: [8]
+	};
+
+	const actual = stringifyObject(obj, {
+		transform: (obj, prop, result) => {
+			if (prop === 'val') {
+				return String(obj[prop] + 1);
+			} else if (prop === 'bar') {
+				return '\'' + result + 'L\'';
+			} else if (obj[prop] === 8) {
+				return 'LOL';
+			}
+			return result;
+		}
+	});
+
+	assert.equal(actual, '{\n\tfoo: {\n\t\tval: 11\n\t},\n\tbar: \'9L\',\n\tbaz: [\n\t\tLOL\n\t]\n}');
+});
+
 it('should not crash with circular references in arrays', () => {
 	const array = [];
 	array.push(array);