feat(helpers): add matchType

index.js

@@ -1,3 +1,5 @@
 module.exports = require('./src').Archetype;
-module.exports.to = require('./src/to');
+
 module.exports.CastError = require('./src').CastError;
+module.exports.matchType = require('./src/helpers/matchType');
+module.exports.to = require('./src/to');

src/helpers/matchType.js

@@ -0,0 +1,24 @@
+'use strict';
+
+/**
+ * Given a mapping from types ('object', 'string', 'boolean', 'undefined',
+ * 'number', 'symbol', or 'function') to functions, returns a function that
+ * takes a `value` and replaces it with the return value from the function
+ * mapped to by `typeof value`.
+ *
+ * Example: `matchType({ 'string': JSON.parse })` will return a function that
+ * takes a value and only calls `JSON.parse()` if that value is a string.
+ *
+ * @param {Object} obj Maps from `typeof` value to function
+ * @return {Function} transforms value based on `typeof`
+ */
+
+module.exports = function(obj) {
+  return function(v) {
+    const type = typeof v;
+    if (typeof obj[type] === 'function') {
+      return obj[type](v);
+    }
+    return v;
+  };
+};

test/helpers.test.js

@@ -0,0 +1,34 @@
+'use strict';
+
+const assert = require('assert');
+const { matchType } = require('../');
+
+describe('matchType', function() {
+  it('works with JSON.parse()', function() {
+    const parse = matchType({ string: JSON.parse });
+
+    const obj = { hello: 'world' };
+
+    // If given a string, will parse it
+    assert.deepEqual(parse(JSON.stringify(obj)), obj);
+
+    // If not, will do nothing
+    assert.strictEqual(parse(obj), obj);
+    assert.strictEqual(parse(null), null);
+    assert.strictEqual(parse(undefined), undefined);
+  });
+
+  it('works with trimming strings', function() {
+    const trim = matchType({ string: str => str.trim() });
+
+    // If given a string, will trim it
+    assert.equal(trim('  abc '), 'abc');
+    assert.equal(trim('abc'), 'abc');
+
+    // If not, will do nothing
+    const obj = {};
+    assert.strictEqual(trim(obj), obj);
+    assert.strictEqual(trim(null), null);
+    assert.strictEqual(trim(undefined), undefined);
+  });
+});