readme driven development

Author: Rich-Harris

README.md

@@ -109,6 +109,18 @@ console.log(vector.magnitude()); // 50
 
 If a function passed to `stringify` returns a truthy value, it's treated as a match.
 
+You can also use custom types with `uneval` by specifying a custom replacer:
+
+```js
+devalue.uneval(vector, (value, uneval) => {
+	if (value instanceof Vector) {
+		return `new Vector(${value.x},${value.y})`;
+	}
+}); // `new Vector(30,40)`
+```
+
+Note that any variables referenced in the resulting JavaScript (like `Vector` in the example above) must be in scope when it runs.
+
 ## Error handling
 
 If `uneval` or `stringify` encounters a function or a non-POJO, it will throw an error. You can find where in the input data the offending value lives by inspecting `error.path`:

src/uneval.js

@@ -15,8 +15,9 @@ const reserved =
 /**
  * Turn a value into the JavaScript that creates an equivalent value
  * @param {any} value
+ * @param {(value: any) => string | void} replacer
  */
-export function uneval(value) {
+export function uneval(value, replacer) {
 	const counts = new Map();
 
 	/** @type {string[]} */

test/test.js

@@ -388,6 +388,11 @@ const fixtures = {
 			value: [instance, instance],
 			js: '(function(a){return [a,a]}(new Custom({answer:42})))',
 			json: '[[1,1],["Custom",2],{"answer":3},42]',
+			replacer: (value, uneval) => {
+				if (value instanceof Custom) {
+					return `new Custom(${uneval(value.value)})`;
+				}
+			},
 			reducers: {
 				Custom: (x) => x instanceof Custom && x.value
 			},