diff --git a/package.json b/package.json
index 83f4979..9756a7b 100644
--- a/package.json
+++ b/package.json
@@ -44,7 +44,15 @@
     "env": {
       "browser": true
     },
-    "extends": "plugin:vue-libs/recommended"
+    "extends": "plugin:vue-libs/recommended",
+    "overrides": [
+      {
+        "files": ["test/*"],
+        "env": {
+          "jest": true
+        }
+      }
+    ]
   },
   "gitHooks": {
     "pre-commit": "lint-staged"
diff --git a/src/index.js b/src/index.js
index 4409bd7..f2dd776 100644
--- a/src/index.js
+++ b/src/index.js
@@ -136,10 +136,14 @@ export default function wrap (Vue, Component) {
       if (!wrapper._isMounted) {
         // initialize attributes
         const syncInitialAttributes = () => {
-          wrapper.props = getInitialProps(camelizedPropsList)
-          hyphenatedPropsList.forEach(key => {
-            syncAttribute(this, key)
-          })
+          const initialProps = getInitialProps(camelizedPropsList, this)
+          const isUndefined = key => initialProps[camelize(key)] === undefined
+          wrapper.props = initialProps
+          hyphenatedPropsList
+            .filter(key => this.hasAttribute(key) || isUndefined(key))
+            .forEach(key => {
+              syncAttribute(this, key)
+            })
         }
 
         if (isInitialized) {
diff --git a/src/utils.js b/src/utils.js
index 0834f03..f2f2601 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -8,10 +8,10 @@ export const hyphenate = str => {
   return str.replace(hyphenateRE, '-$1').toLowerCase()
 }
 
-export function getInitialProps (propsList) {
+export function getInitialProps (propsList, el) {
   const res = {}
   propsList.forEach(key => {
-    res[key] = undefined
+    res[key] = el[key]
   })
   return res
 }
diff --git a/test/fixtures/mounting-manually.html b/test/fixtures/mounting-manually.html
new file mode 100644
index 0000000..7ce426e
--- /dev/null
+++ b/test/fixtures/mounting-manually.html
@@ -0,0 +1,23 @@
+<script src="../../node_modules/vue/dist/vue.js"></script>
+<script type="module">
+import wrap from '../../src/index.js'
+
+window.MyElement = wrap(Vue, {
+  template: `<div>{{ foo }} {{ someProp }}</div>`,
+  props: {
+    foo: {
+      type: Number,
+      default: 123
+    },
+    'some-prop': {
+      type: String,
+      default: 'bar'
+    }
+  },
+  mounted () {
+    console.log(`mounted with foo: ${this.foo}`)
+    console.log(`mounted with someProp: ${this.someProp}`)
+  }
+})
+customElements.define('my-element', window.MyElement)
+</script>
diff --git a/test/test.js b/test/test.js
index 9423d7a..e098cb6 100644
--- a/test/test.js
+++ b/test/test.js
@@ -1,5 +1,7 @@
 const launchPage = require('./setup')
 
+/* global el, els, MyElement */
+
 test('properties', async () => {
   const { page } = await launchPage(`properties`)
 
@@ -16,9 +18,9 @@ test('properties', async () => {
     el.foo = 234
     el.someProp = 'lol'
   })
-  const newFoo = await page.evaluate(()  => el.vueComponent.foo)
+  const newFoo = await page.evaluate(() => el.vueComponent.foo)
   expect(newFoo).toBe(234)
-  const newBar = await page.evaluate(()  => el.vueComponent.someProp)
+  const newBar = await page.evaluate(() => el.vueComponent.someProp)
   expect(newBar).toBe('lol')
 })
 
@@ -130,3 +132,26 @@ test('async', async () => {
     return document.querySelectorAll('my-element')[2].shadowRoot.textContent
   })).toMatch(`456 bar`)
 })
+
+test('mounting manually', async () => {
+  const { page, logs } = await launchPage(`mounting-manually`)
+
+  // mounting programmatically
+  await page.evaluate(() => {
+    window.el = new MyElement()
+    window.el.foo = 234
+    window.el.setAttribute('some-prop', 'lol')
+    window.el.someProp = 'ignored as attribute takes precedence'
+    document.body.appendChild(window.el)
+  })
+
+  // props
+  const foo = await page.evaluate(() => el.vueComponent.foo)
+  expect(foo).toBe(234)
+  const bar = await page.evaluate(() => el.vueComponent.someProp)
+  expect(bar).toBe('lol')
+
+  // lifecycle
+  expect(logs).toContain('mounted with foo: 234')
+  expect(logs).toContain('mounted with someProp: lol')
+})