Add support for path matcher function

index.js

@@ -10,9 +10,10 @@ module.exports = (filename, opts = {}) => {
 
 	return new Promise(resolve => {
 		(function find(dir) {
-			locatePath(filenames, {cwd: dir}).then(file => {
+			const locating = typeof filename === 'function' ? Promise.resolve(filename(dir)) : locatePath(filenames, {cwd: dir});
+			locating.then(file => {
 				if (file) {
-					resolve(path.join(dir, file));
+					resolve(path.resolve(dir, file));
 				} else if (dir === root) {
 					resolve(null);
 				} else {
@@ -31,10 +32,10 @@ module.exports.sync = (filename, opts = {}) => {
 
 	// eslint-disable-next-line no-constant-condition
 	while (true) {
-		const file = locatePath.sync(filenames, {cwd: dir});
+		const file = typeof filename === 'function' ? filename(dir) : locatePath.sync(filenames, {cwd: dir});
 
 		if (file) {
-			return path.join(dir, file);
+			return path.resolve(dir, file);
 		}
 
 		if (dir === root) {

package.json

@@ -44,6 +44,8 @@
 	},
 	"devDependencies": {
 		"ava": "*",
+		"is-path-inside": "^2.0.0",
+		"pify": "^4.0.0",
 		"tempy": "^0.2.1",
 		"xo": "*"
 	}

readme.md

@@ -47,6 +47,12 @@ const findUp = require('find-up');
 
 	console.log(await findUp(['rainbow.png', 'unicorn.png']));
 	//=> '/Users/sindresorhus/unicorn.png'
+
+	console.log(await findUp(dir => Promise.resolve('unicorn.png')));
+	//=> '/Users/sindresorhus/unicorn.png'
+
+	console.log(await findUp(dir => dir));
+	//=> '/Users/sindresorhus'
 })();
 ```
 
@@ -71,9 +77,11 @@ Returns the first filepath found (by respecting the order) or `null`.
 
 #### filename
 
-Type: `string`
+Type: `string` `Function`
+
+Filename of the file to find, or a custom matcher function to be called with each directory until it returns a filepath to stop the search or the root directory has been reached and nothing was found. When using a matcher function, if you want to check whether a file exists, use [`fs.access()`](https://nodejs.org/api/fs.html#fs_fs_access_path_mode_callback) - this is done automatically when `filename` is a string.
 
-Filename of the file to find.
+When using async mode, `filename` may optionally be an `async` function or return a `Promise` for the filepath.
 
 #### options
 

test.js

@@ -1,6 +1,8 @@
 import fs from 'fs';
 import path from 'path';
 import test from 'ava';
+import isPathInside from 'is-path-inside';
+import pify from 'pify';
 import tempy from 'tempy';
 import m from '.';
 
@@ -244,3 +246,87 @@ test('sync (not found, custom cwd)', t => {
 
 	t.is(filePath, null);
 });
+
+test('async (custom function)', async t => {
+	const cwd = process.cwd();
+
+	t.is(await m(dir => {
+		t.is(dir, cwd);
+		return dir;
+	}), cwd);
+
+	t.is(await m(() => {
+		return '.';
+	}), cwd);
+
+	t.is(await m(() => {
+		return Promise.resolve('foo.txt');
+	}), path.join(cwd, 'foo.txt'));
+
+	t.is(await m(() => {
+		return '..';
+	}), path.join(cwd, '..'));
+
+	t.is(await m(dir => {
+		return (dir !== cwd) && dir;
+	}), path.join(cwd, '..'));
+
+	t.is(await m(dir => {
+		return (dir !== cwd) && 'foo.txt';
+	}), path.join(cwd, '..', 'foo.txt'));
+
+	const {root} = path.parse(cwd);
+	const visited = new Set();
+	t.is(await m(async dir => {
+		t.is(typeof dir, 'string');
+		const stat = await pify(fs.stat)(dir);
+		t.true(stat.isDirectory());
+		t.true((dir === cwd) || isPathInside(cwd, dir));
+		t.false(visited.has(dir));
+		visited.add(dir);
+	}), null);
+	t.true(visited.has(cwd));
+	t.true(visited.has(root));
+});
+
+test('sync (custom function)', t => {
+	const cwd = process.cwd();
+
+	t.is(m.sync(dir => {
+		t.is(dir, cwd);
+		return dir;
+	}), cwd);
+
+	t.is(m.sync(() => {
+		return '.';
+	}), cwd);
+
+	t.is(m.sync(() => {
+		return 'foo.txt';
+	}), path.join(cwd, 'foo.txt'));
+
+	t.is(m.sync(() => {
+		return '..';
+	}), path.join(cwd, '..'));
+
+	t.is(m.sync(dir => {
+		return (dir !== cwd) && dir;
+	}), path.join(cwd, '..'));
+
+	t.is(m.sync(dir => {
+		return (dir !== cwd) && 'foo.txt';
+	}), path.join(cwd, '..', 'foo.txt'));
+
+	const {root} = path.parse(cwd);
+	const visited = new Set();
+	t.is(m.sync(dir => {
+		t.is(typeof dir, 'string');
+		const stat = fs.statSync(dir);
+		t.true(stat.isDirectory());
+		t.true((dir === cwd) || isPathInside(cwd, dir));
+		t.false(visited.has(dir));
+		visited.add(dir);
+	}), null);
+	t.true(visited.has(cwd));
+	t.true(visited.has(root));
+});