diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index ab84a1a..23afef4 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -2,7 +2,7 @@ name: Release
 
 on:
   push:
-    branches: master
+    branches: [master, next, next-major, beta, alpha]
 
 jobs:
   release:
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 3cf7d1b..1da5c2d 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -4,7 +4,7 @@ on:
   push:
     branches: [develop]
   pull_request:
-    branches: [master, develop]
+    branches: [develop, master, next, next-major, beta, alpha]
 
 jobs:
   test:
diff --git a/.xo-config b/.xo-config
deleted file mode 100644
index 537ca57..0000000
--- a/.xo-config
+++ /dev/null
@@ -1,11 +0,0 @@
-{
-	"rules": {
-		"comma-dangle": ["error", "always-multiline"]
-	},
-	"overrides": [
-		{
-			"files": "test/*",
-			"env": "jest"
-		}
-	]
-}
\ No newline at end of file
diff --git a/LICENSE b/LICENSE
index 957a161..dff8850 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2020 Hiroki Osame
+Copyright (c) 2020 Hiroki Osame <hiroki.osame@gmail.com>
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
diff --git a/README.md b/README.md
index b9dddf3..474229c 100644
--- a/README.md
+++ b/README.md
@@ -1,13 +1,93 @@
-# Pkg Name <a href="https://npm.im/pkg-name"><img src="https://badgen.net/npm/v/pkg-name"></a> <a href="https://npm.im/pkg-name"><img src="https://badgen.net/npm/dm/pkg-name"></a> <a href="https://packagephobia.now.sh/result?p=pkg-name"><img src="https://packagephobia.now.sh/badge?p=pkg-name"></a> <a href="https://bundlephobia.com/result?p=pkg-name"><img src="https://badgen.net/bundlephobia/minzip/pkg-name"></a>
+# postcss-custom-properties-transformer [![Latest version](https://badgen.net/npm/v/postcss-custom-properties-transformer)](https://npm.im/postcss-custom-properties-transformer) [![Monthly downloads](https://badgen.net/npm/dm/postcss-custom-properties-transformer)](https://npm.im/postcss-custom-properties-transformer) [![Install size](https://packagephobia.now.sh/badge?p=postcss-custom-properties-transformer)](https://packagephobia.now.sh/result?p=postcss-custom-properties-transformer)
 
-Description
+PostCSS plugin to transform [CSS custom properties](https://developer.mozilla.org/en-US/docs/Web/CSS/--*)
 
-## :raising_hand: Why?
+## 🙋‍♂️ Why?
 
+Transform CSS custom properties to...
 
-## :rocket: Install
+- **🔥 Scope to package** Namespace them to your library to prevent collision with other custom properties
+- **👌 Scope to file** Scope to a file to prevent collision with other files
+- **🐥 Minify** Shorten long custom properties via hashing
+- **🤬 Obfuscate** Mangle custom-property names to deter reverse-engineering
+
+## 🚀 Install
 ```sh
-npm i pkg-name
+npm i -D postcss postcss-custom-properties-transformer
+```
+
+## 🚦 Quick Setup
+
+Add `postcss-custom-properties-transformer` to your PostCSS configuration (eg. `postcss.config.js`) and pass in a `transformer` function.
+
+> Warning: this plugin doesn't validate custom properties. [Make sure to not use invalid characters (eg. period)](https://stackoverflow.com/a/42311038)
+
+```diff
+module.exports = {
+     plugins: [
+
++        // Insert above plugins that read custom properties
++        require('postcss-custom-properties-transformer')({
++            transformer({ property }) {
++                // Prefixing all custom properties with 'APP-'
++                return 'APP-' + property;
++            }
++        }),
+
+         require('postcss-preset-env')()
+     ]
+};
+```
+
+## 👨🏻‍🏫 Examples
+
+### Namespace with package meta
+If you have a CSS library, you can scope your custom properties to every release so that multiple versions of the library used in the same app doesn't yield any collisions.
+
+```js
+const pkg = require('./package.json')
+require('postcss-custom-properties-transformer')({
+    transformer({ property }) {
+        return `${pkg.name}-${pkg.version}-${property}`;
+    }
+})
+```
+
+### Hash custom properties
+If you want to hash your custom properties to shorten/obfuscate them, pass in a hashing algorithm of your choice.
+
+This demo uses a 6-character truncated MD5 hash; MD5 and the SHA-family has [statistically good uniform distribution](https://stackoverflow.com/questions/8184941/uniform-distribution-of-truncated-md5) and can be truncated.
+
+However, note that the shorter the hash, the higher the collision rate. There will be a warning if a collision is detected.
+
+```js
+const crypto = require('crypto');
+const md5 = string => crypto.createHash('md5').update(string).digest('hex');
+
+require('postcss-custom-properties-transformer')({
+    transformer({ property }) {
+        return md5(property).slice(0, 6);
+    }
+})
 ```
 
+### Advanced transformations
+If you want to do something more complex—such as discriminate between global and local custom properties (eg. theme variables).
+
+```js
+require('postcss-custom-properties-transformer')({
+    transformer({ property }) {
+        if (property.startsWith('theme-')) {
+            return property;
+        }
+        return hash(property);
+    }
+})
+```
 
+## ⚙️ Options
+- `transformer(data)` `<Function>`
+  - `data` `<Object>`
+    - `property` `<String>` - The custom property name that's being transformed. The `--` prefix is omitted
+    - `filepath` `<String>` - The path to the file where the custom property is being transformed
+    - `css` `<String>` - The entire CSS code of the file
diff --git a/lib/index.js b/lib/index.js
index e69de29..4b96f7f 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -0,0 +1,65 @@
+const postcss = require('postcss');
+const assert = require('assert');
+const processPropertyValue = require('./process-property-value');
+
+const {stringify: string} = JSON;
+
+// From postcss-custom-properties
+// https://github.com/postcss/postcss-custom-properties/blob/1e1ef6b/src/lib/transform-properties.js#L37
+const customPropertyPtrn = /^--[A-z][\w-]*$/;
+const varFnPtrn = /(^|[^\w-])var\([\W\w]+\)/;
+const name = 'postcss-custom-properties-transformer';
+
+function markHashUsed(map, propertyName, originalPropertyName) {
+	if (!map.has(propertyName)) {
+		map.set(propertyName, originalPropertyName);
+		return;
+	}
+
+	const previousPropName = map.get(propertyName);
+	if (originalPropertyName !== previousPropName) {
+		console.warn(`[${name}] Collision: property name ${string(propertyName)} was generated from input ${string(originalPropertyName)} but was already generated from input ${string(previousPropName)}`);
+	}
+}
+
+const createTransformer = transformer => {
+	const usedPropertyNames = new Map();
+
+	return (customProperty, data) => {
+		data.property = customProperty.slice(2);
+
+		const newPropertyName = '--' + transformer(data);
+
+		markHashUsed(usedPropertyNames, newPropertyName, customProperty);
+
+		return newPropertyName;
+	};
+};
+
+const customPropertiesTransformer = postcss.plugin(name, options => {
+	assert(options && options.transformer, `[${name}] a transformer must be passed in`);
+	assert(typeof options.transformer === 'function', `[${name}] transformer must be a function`);
+
+	const transformer = createTransformer(options.transformer);
+
+	return root => {
+		const data = Object.create({
+			filepath: root.source.input.file,
+			css: root.source.input.css,
+		});
+
+		root.walkDecls(decl => {
+			// Custom property declaration
+			if (customPropertyPtrn.test(decl.prop)) {
+				decl.prop = transformer(decl.prop, data);
+			}
+
+			// Custom property usage
+			if (varFnPtrn.test(decl.value)) {
+				decl.value = processPropertyValue(decl.value, customProperty => transformer(customProperty, data));
+			}
+		});
+	};
+});
+
+module.exports = customPropertiesTransformer;
diff --git a/lib/process-property-value.js b/lib/process-property-value.js
new file mode 100644
index 0000000..6a122fe
--- /dev/null
+++ b/lib/process-property-value.js
@@ -0,0 +1,29 @@
+const {parse} = require('postcss-values-parser');
+
+function transformAstVars(root, transformer) {
+	if (!root.nodes) {
+		return;
+	}
+
+	root.nodes.forEach(node => {
+		if (node.isVar) {
+			node.nodes.forEach(node2 => {
+				if (node2.isVariable) {
+					node2.value = transformer(node2.value);
+				}
+			});
+		} else {
+			transformAstVars(node, transformer);
+		}
+	});
+}
+
+function processPropertyValue(value, cb) {
+	const parsed = parse(value);
+
+	transformAstVars(parsed, cb);
+
+	return parsed.toString();
+}
+
+module.exports = processPropertyValue;
diff --git a/package-lock.json b/package-lock.json
index 298d580..76a2c36 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,5 +1,5 @@
 {
-  "name": "NODE-PACKAGE-TEMPLATE",
+  "name": "postcss-custom-properties-transformer",
   "version": "0.0.0-semantic-release",
   "lockfileVersion": 1,
   "requires": true,
@@ -1890,8 +1890,7 @@
     "color-name": {
       "version": "1.1.4",
       "resolved": "https://registry.npmjs.org/color-name/-/color-name-1.1.4.tgz",
-      "integrity": "sha512-dOy+3AuW3a2wNbZHIuMZpTcgjGuLU/uBL/ubcZF9OXbDo8ff4O8yVp5Bf0efS8uEoYo5q4Fx7dY9OgQGXgAsQA==",
-      "dev": true
+      "integrity": "sha512-dOy+3AuW3a2wNbZHIuMZpTcgjGuLU/uBL/ubcZF9OXbDo8ff4O8yVp5Bf0efS8uEoYo5q4Fx7dY9OgQGXgAsQA=="
     },
     "combined-stream": {
       "version": "1.0.8",
@@ -2548,8 +2547,7 @@
     "escape-string-regexp": {
       "version": "1.0.5",
       "resolved": "https://registry.npmjs.org/escape-string-regexp/-/escape-string-regexp-1.0.5.tgz",
-      "integrity": "sha1-G2HAViGQqN/2rjuyzwIAyhMLhtQ=",
-      "dev": true
+      "integrity": "sha1-G2HAViGQqN/2rjuyzwIAyhMLhtQ="
     },
     "escodegen": {
       "version": "1.14.3",
@@ -4622,6 +4620,14 @@
         "unc-path-regex": "^0.1.2"
       }
     },
+    "is-url-superb": {
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/is-url-superb/-/is-url-superb-3.0.0.tgz",
+      "integrity": "sha512-3faQP+wHCGDQT1qReM5zCPx2mxoal6DzbzquFlCYJLWyy4WPTved33ea2xFbX37z4NoriEwZGIYhFtx8RUB5wQ==",
+      "requires": {
+        "url-regex": "^5.0.0"
+      }
+    },
     "is-windows": {
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/is-windows/-/is-windows-1.0.2.tgz",
@@ -6530,6 +6536,83 @@
       "integrity": "sha1-AerA/jta9xoqbAL+q7jB/vfgDqs=",
       "dev": true
     },
+    "postcss": {
+      "version": "7.0.32",
+      "resolved": "https://registry.npmjs.org/postcss/-/postcss-7.0.32.tgz",
+      "integrity": "sha512-03eXong5NLnNCD05xscnGKGDZ98CyzoqPSMjOe6SuoQY7Z2hIj0Ld1g/O/UQRuOle2aRtiIRDg9tDcTGAkLfKw==",
+      "requires": {
+        "chalk": "^2.4.2",
+        "source-map": "^0.6.1",
+        "supports-color": "^6.1.0"
+      },
+      "dependencies": {
+        "ansi-styles": {
+          "version": "3.2.1",
+          "resolved": "https://registry.npmjs.org/ansi-styles/-/ansi-styles-3.2.1.tgz",
+          "integrity": "sha512-VT0ZI6kZRdTh8YyJw3SMbYm/u+NqfsAxEpWO0Pf9sq8/e94WxxOpPKx9FR1FlyCtOVDNOQ+8ntlqFxiRc+r5qA==",
+          "requires": {
+            "color-convert": "^1.9.0"
+          }
+        },
+        "chalk": {
+          "version": "2.4.2",
+          "resolved": "https://registry.npmjs.org/chalk/-/chalk-2.4.2.tgz",
+          "integrity": "sha512-Mti+f9lpJNcwF4tWV8/OrTTtF1gZi+f8FqlyAdouralcFWFQWF2+NgCHShjkCb+IFBLq9buZwE1xckQU4peSuQ==",
+          "requires": {
+            "ansi-styles": "^3.2.1",
+            "escape-string-regexp": "^1.0.5",
+            "supports-color": "^5.3.0"
+          },
+          "dependencies": {
+            "supports-color": {
+              "version": "5.5.0",
+              "resolved": "https://registry.npmjs.org/supports-color/-/supports-color-5.5.0.tgz",
+              "integrity": "sha512-QjVjwdXIt408MIiAqCX4oUKsgU2EqAGzs2Ppkm4aQYbjm+ZEWEcW4SfFNTr4uMNZma0ey4f5lgLrkB0aX0QMow==",
+              "requires": {
+                "has-flag": "^3.0.0"
+              }
+            }
+          }
+        },
+        "color-convert": {
+          "version": "1.9.3",
+          "resolved": "https://registry.npmjs.org/color-convert/-/color-convert-1.9.3.tgz",
+          "integrity": "sha512-QfAUtd+vFdAtFQcC8CCyYt1fYWxSqAiK2cSD6zDB8N3cpsEBAvRxp9zOGg6G/SHHJYAT88/az/IuDGALsNVbGg==",
+          "requires": {
+            "color-name": "1.1.3"
+          }
+        },
+        "color-name": {
+          "version": "1.1.3",
+          "resolved": "https://registry.npmjs.org/color-name/-/color-name-1.1.3.tgz",
+          "integrity": "sha1-p9BVi9icQveV3UIyj3QIMcpTvCU="
+        },
+        "has-flag": {
+          "version": "3.0.0",
+          "resolved": "https://registry.npmjs.org/has-flag/-/has-flag-3.0.0.tgz",
+          "integrity": "sha1-tdRU3CGZriJWmfNGfloH87lVuv0="
+        },
+        "supports-color": {
+          "version": "6.1.0",
+          "resolved": "https://registry.npmjs.org/supports-color/-/supports-color-6.1.0.tgz",
+          "integrity": "sha512-qe1jfm1Mg7Nq/NSh6XE24gPXROEVsWHxC1LIx//XNlD9iw7YZQGjZNjYN7xGaEG6iKdA8EtNFW6R0gjnVXp+wQ==",
+          "requires": {
+            "has-flag": "^3.0.0"
+          }
+        }
+      }
+    },
+    "postcss-values-parser": {
+      "version": "3.2.1",
+      "resolved": "https://registry.npmjs.org/postcss-values-parser/-/postcss-values-parser-3.2.1.tgz",
+      "integrity": "sha512-SQ7/88VE9LhJh9gc27/hqnSU/aZaREVJcRVccXBmajgP2RkjdJzNyH/a9GCVMI5nsRhT0jC5HpUMwfkz81DVVg==",
+      "requires": {
+        "color-name": "^1.1.4",
+        "is-url-superb": "^3.0.0",
+        "postcss": "^7.0.5",
+        "url-regex": "^5.0.0"
+      }
+    },
     "prelude-ls": {
       "version": "1.1.2",
       "resolved": "https://registry.npmjs.org/prelude-ls/-/prelude-ls-1.1.2.tgz",
@@ -7028,6 +7111,15 @@
         "inherits": "^2.0.1"
       }
     },
+    "rollup": {
+      "version": "2.26.8",
+      "resolved": "https://registry.npmjs.org/rollup/-/rollup-2.26.8.tgz",
+      "integrity": "sha512-li9WaJYc5z9WzV1jhZbPQCrsOpGNsI+Li1qyrn5n745ZNSnlkRlBtj1Hs+Z0Dc2N1+P7HT34UKAEASqN9Th8cg==",
+      "dev": true,
+      "requires": {
+        "fsevents": "~2.1.2"
+      }
+    },
     "rsvp": {
       "version": "4.8.5",
       "resolved": "https://registry.npmjs.org/rsvp/-/rsvp-4.8.5.tgz",
@@ -7476,8 +7568,7 @@
     "source-map": {
       "version": "0.6.1",
       "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.6.1.tgz",
-      "integrity": "sha512-UjgapumWlbMhkBgzT7Ykc5YXUT46F0iKu8SGXq0bcwP5dz/h0Plj6enJqjz1Zbq2l5WaqYnrVbwWOWMyF3F47g==",
-      "dev": true
+      "integrity": "sha512-UjgapumWlbMhkBgzT7Ykc5YXUT46F0iKu8SGXq0bcwP5dz/h0Plj6enJqjz1Zbq2l5WaqYnrVbwWOWMyF3F47g=="
     },
     "source-map-resolve": {
       "version": "0.5.3",
@@ -7934,6 +8025,11 @@
         "setimmediate": "^1.0.4"
       }
     },
+    "tlds": {
+      "version": "1.209.0",
+      "resolved": "https://registry.npmjs.org/tlds/-/tlds-1.209.0.tgz",
+      "integrity": "sha512-KVsZ1NSpBodpo42/JIwTyau7SqUxV/qQMp2epSDPa99885LpHWLaVCCt8CWzGe4X5YIVNr+b6bUys9e9eEb5OA=="
+    },
     "tmpl": {
       "version": "1.0.4",
       "resolved": "https://registry.npmjs.org/tmpl/-/tmpl-1.0.4.tgz",
@@ -8284,6 +8380,22 @@
         "prepend-http": "^2.0.0"
       }
     },
+    "url-regex": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/url-regex/-/url-regex-5.0.0.tgz",
+      "integrity": "sha512-O08GjTiAFNsSlrUWfqF1jH0H1W3m35ZyadHrGv5krdnmPPoxP27oDTqux/579PtaroiSGm5yma6KT1mHFH6Y/g==",
+      "requires": {
+        "ip-regex": "^4.1.0",
+        "tlds": "^1.203.0"
+      },
+      "dependencies": {
+        "ip-regex": {
+          "version": "4.1.0",
+          "resolved": "https://registry.npmjs.org/ip-regex/-/ip-regex-4.1.0.tgz",
+          "integrity": "sha512-pKnZpbgCTfH/1NLIlOduP/V+WRXzC2MOz3Qo8xmxk8C5GudJLgK5QyLVXOSWy3ParAH7Eemurl3xjv/WXYFvMA=="
+        }
+      }
+    },
     "use": {
       "version": "3.1.1",
       "resolved": "https://registry.npmjs.org/use/-/use-3.1.1.tgz",
diff --git a/package.json b/package.json
index 4a48fbc..a88a2a9 100644
--- a/package.json
+++ b/package.json
@@ -1,15 +1,20 @@
 {
-  "name": "NODE-PACKAGE-TEMPLATE",
+  "name": "postcss-custom-properties-transformer",
   "version": "0.0.0-semantic-release",
-  "description": "Template for Node package repos",
-  "keywords": [],
+  "description": "PostCSS plugin to transform custom properties",
+  "keywords": [
+    "postcss",
+    "plugin",
+    "custom properties",
+    "transformer"
+  ],
   "license": "MIT",
-  "repository": "privatenumber/NODE-PACKAGE-TEMPLATE",
+  "repository": "privatenumber/postcss-custom-properties-transformer",
   "author": "Hiroki Osame <hiroki.osame@gmail.com>",
   "files": [
-    "index.js"
+    "lib"
   ],
-  "main": "index.js",
+  "main": "lib/index.js",
   "scripts": {
     "test": "jest",
     "lint": "xo"
@@ -20,12 +25,19 @@
     }
   },
   "lint-staged": {
-    "*.js": "xo"
+    "*.js": [
+      "xo",
+      "jest --bail --findRelatedTests"
+    ]
   },
   "devDependencies": {
     "husky": "^4.2.5",
     "jest": "^26.4.2",
     "lint-staged": "^10.2.13",
     "xo": "^0.33.0"
+  },
+  "dependencies": {
+    "postcss": "^7.0.32",
+    "postcss-values-parser": "^3.2.1"
   }
 }
diff --git a/test/__snapshots__/index.spec.js.snap b/test/__snapshots__/index.spec.js.snap
new file mode 100644
index 0000000..f918968
--- /dev/null
+++ b/test/__snapshots__/index.spec.js.snap
@@ -0,0 +1,169 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`transformer function custom hash - scoped to file 1`] = `
+Array [
+  "
+	.button {
+		--ae7621: red;
+		--b682a4: 24px;
+		background: var(--ae7621);
+		border-color: var(--ae7621);
+		font-size: var(--b682a4);
+	}
+
+	.input {
+		--ae7621: red;
+		--b682a4: 24px;
+		background: var(--ae7621);
+		border-color: var(--ae7621);
+		font-size: var(--b682a4);
+		border: 1px solid var(--ae7621);
+	}
+	",
+  "
+	:root {
+		--199650: red;
+		--29d9c5: 24px;
+		--d85b32: calc(var(--29d9c5) * 2);
+	}
+	.section {
+		background: var(--199650);
+		border-color: var(--199650);
+		font-size: var(--29d9c5);
+	}
+
+	.heading {
+		background: var(--199650);
+		border-color: var(--199650);
+		font-size: var(--29d9c5);
+		border: 1px solid var(--199650);
+	}
+	",
+]
+`;
+
+exports[`transformer function custom hash 1`] = `
+Array [
+  "
+	.button {
+		--70dda5: red;
+		--9b1d0a: 24px;
+		background: var(--70dda5);
+		border-color: var(--70dda5);
+		font-size: var(--9b1d0a);
+	}
+
+	.input {
+		--70dda5: red;
+		--9b1d0a: 24px;
+		background: var(--70dda5);
+		border-color: var(--70dda5);
+		font-size: var(--9b1d0a);
+		border: 1px solid var(--70dda5);
+	}
+	",
+  "
+	:root {
+		--70dda5: red;
+		--9b1d0a: 24px;
+		--defe4c: calc(var(--9b1d0a) * 2);
+	}
+	.section {
+		background: var(--70dda5);
+		border-color: var(--70dda5);
+		font-size: var(--9b1d0a);
+	}
+
+	.heading {
+		background: var(--70dda5);
+		border-color: var(--70dda5);
+		font-size: var(--9b1d0a);
+		border: 1px solid var(--70dda5);
+	}
+	",
+]
+`;
+
+exports[`transformer function emoji namespace 1`] = `
+Array [
+  "
+	.button {
+		--🔥-color: red;
+		--🔥-font-size: 24px;
+		background: var(--🔥-color);
+		border-color: var(--🔥-color);
+		font-size: var(--🔥-font-size);
+	}
+
+	.input {
+		--🔥-color: red;
+		--🔥-font-size: 24px;
+		background: var(--🔥-color);
+		border-color: var(--🔥-color);
+		font-size: var(--🔥-font-size);
+		border: 1px solid var(--🔥-color);
+	}
+	",
+  "
+	:root {
+		--🔥-color: red;
+		--🔥-font-size: 24px;
+		--🔥-large-font-size: calc(var(--🔥-font-size) * 2);
+	}
+	.section {
+		background: var(--🔥-color);
+		border-color: var(--🔥-color);
+		font-size: var(--🔥-font-size);
+	}
+
+	.heading {
+		background: var(--🔥-color);
+		border-color: var(--🔥-color);
+		font-size: var(--🔥-font-size);
+		border: 1px solid var(--🔥-color);
+	}
+	",
+]
+`;
+
+exports[`transformer function namespace 1`] = `
+Array [
+  "
+	.button {
+		--app-color: red;
+		--app-font-size: 24px;
+		background: var(--app-color);
+		border-color: var(--app-color);
+		font-size: var(--app-font-size);
+	}
+
+	.input {
+		--app-color: red;
+		--app-font-size: 24px;
+		background: var(--app-color);
+		border-color: var(--app-color);
+		font-size: var(--app-font-size);
+		border: 1px solid var(--app-color);
+	}
+	",
+  "
+	:root {
+		--app-color: red;
+		--app-font-size: 24px;
+		--app-large-font-size: calc(var(--app-font-size) * 2);
+	}
+	.section {
+		background: var(--app-color);
+		border-color: var(--app-color);
+		font-size: var(--app-font-size);
+	}
+
+	.heading {
+		background: var(--app-color);
+		border-color: var(--app-color);
+		font-size: var(--app-font-size);
+		border: 1px solid var(--app-color);
+	}
+	",
+]
+`;
diff --git a/test/fixtures.js b/test/fixtures.js
new file mode 100644
index 0000000..d9278aa
--- /dev/null
+++ b/test/fixtures.js
@@ -0,0 +1,41 @@
+const fixtures = {
+	'file-a.css': `
+	.button {
+		--color: red;
+		--font-size: 24px;
+		background: var(--color);
+		border-color: var(--color);
+		font-size: var(--font-size);
+	}
+
+	.input {
+		--color: red;
+		--font-size: 24px;
+		background: var(--color);
+		border-color: var(--color);
+		font-size: var(--font-size);
+		border: 1px solid var(--color);
+	}
+	`,
+	'file-b.css': `
+	:root {
+		--color: red;
+		--font-size: 24px;
+		--large-font-size: calc(var(--font-size) * 2);
+	}
+	.section {
+		background: var(--color);
+		border-color: var(--color);
+		font-size: var(--font-size);
+	}
+
+	.heading {
+		background: var(--color);
+		border-color: var(--color);
+		font-size: var(--font-size);
+		border: 1px solid var(--color);
+	}
+	`,
+};
+
+module.exports = fixtures;
diff --git a/test/index.spec.js b/test/index.spec.js
index e69de29..3f2d791 100644
--- a/test/index.spec.js
+++ b/test/index.spec.js
@@ -0,0 +1,71 @@
+const {transform, md5} = require('./utils');
+const fixtures = require('./fixtures');
+
+describe('error handling', () => {
+	test('no options', () => {
+		expect(() => transform(fixtures)).toThrowError('[postcss-custom-properties-transformer] a transformer must be passed in');
+	});
+
+	test('no transformer', () => {
+		expect(() => transform(fixtures, {})).toThrowError('[postcss-custom-properties-transformer] a transformer must be passed in');
+	});
+
+	test('invalid transformer', () => {
+		expect(() => transform(fixtures, {
+			transformer: 123,
+		})).toThrowError('[postcss-custom-properties-transformer] transformer must be a function');
+	});
+
+	test('transformer collision warning', async () => {
+		const spy = jest.spyOn(global.console, 'warn').mockImplementation();
+		await transform(fixtures, {
+			transformer() {
+				return 'constant-value';
+			},
+		});
+		expect(spy).toHaveBeenCalledWith('[postcss-custom-properties-transformer] Collision: property name "--constant-value" was generated from input "--font-size" but was already generated from input "--color"');
+		spy.mockRestore();
+	});
+});
+
+describe('transformer function', () => {
+	test('namespace', async () => {
+		const output = await transform(fixtures, {
+			transformer({property}) {
+				return `app-${property}`;
+			},
+		});
+
+		expect(output).toMatchSnapshot();
+	});
+
+	test('emoji namespace', async () => {
+		const output = await transform(fixtures, {
+			transformer({property}) {
+				return `🔥-${property}`;
+			},
+		});
+
+		expect(output).toMatchSnapshot();
+	});
+
+	test('custom hash', async () => {
+		const output = await transform(fixtures, {
+			transformer({property}) {
+				return md5(property).slice(0, 6);
+			},
+		});
+
+		expect(output).toMatchSnapshot();
+	});
+
+	test('custom hash - scoped to file', async () => {
+		const output = await transform(fixtures, {
+			transformer({property, filepath}) {
+				return md5(filepath + property).slice(0, 6);
+			},
+		});
+
+		expect(output).toMatchSnapshot();
+	});
+});
diff --git a/test/utils.js b/test/utils.js
new file mode 100644
index 0000000..a909fca
--- /dev/null
+++ b/test/utils.js
@@ -0,0 +1,23 @@
+const crypto = require('crypto');
+const postcss = require('postcss');
+const postcssCustomPropertiesTransformer = require('..');
+
+const md5 = string => crypto.createHash('md5').update(string).digest('hex');
+
+const transform = (cssFiles, options) => {
+	const processor = postcss([
+		postcssCustomPropertiesTransformer(options),
+	]);
+
+	return Promise.all(Object.entries(cssFiles).map(async ([name, cssFile]) => {
+		const result = await processor.process(cssFile, {
+			from: '/' + name,
+		});
+		return result.css;
+	}));
+};
+
+module.exports = {
+	transform,
+	md5,
+};
diff --git a/xo.config.js b/xo.config.js
new file mode 100644
index 0000000..041cdf5
--- /dev/null
+++ b/xo.config.js
@@ -0,0 +1,11 @@
+module.exports = {
+	rules: {
+		'comma-dangle': ['error', 'always-multiline'],
+	},
+	overrides: [
+		{
+			files: 'test/*',
+			env: 'jest',
+		},
+	],
+};