--- /dev/null
+'use strict';
+
+var isObject = require('is-extendable');
+var union = require('arr-union');
+var get = require('get-value');
+var set = require('set-value');
+
+module.exports = function unionValue(obj, prop, value) {
+ if (!isObject(obj)) {
+ throw new TypeError('union-value expects the first argument to be an object.');
+ }
+
+ if (typeof prop !== 'string') {
+ throw new TypeError('union-value expects `prop` to be a string.');
+ }
+
+ var arr = arrayify(get(obj, prop));
+ set(obj, prop, union(arr, arrayify(value)));
+ return obj;
+};
+
+function arrayify(val) {
+ if (val === null || typeof val === 'undefined') {
+ return [];
+ }
+ if (Array.isArray(val)) {
+ return val;
+ }
+ return [val];
+}