diff --git a/packages/core/build.mjs b/packages/core/build.mjs
index 9d1da1994da..0f283af1c6d 100644
--- a/packages/core/build.mjs
+++ b/packages/core/build.mjs
@@ -14,7 +14,7 @@ const banner = `/**
 const options = {
   banner: { js: banner },
   bundle: true,
-  entryPoints: ['src/index.js'],
+  entryPoints: ['src/index.mjs'],
   format: 'esm',
   outfile: 'dist/index.mjs',
   external: ["@freesewing"],
diff --git a/packages/core/src/attributes.js b/packages/core/src/attributes.mjs
similarity index 100%
rename from packages/core/src/attributes.js
rename to packages/core/src/attributes.mjs
diff --git a/packages/core/src/design.js b/packages/core/src/design.mjs
similarity index 98%
rename from packages/core/src/design.js
rename to packages/core/src/design.mjs
index 2e1504cd072..74c19cf3934 100644
--- a/packages/core/src/design.js
+++ b/packages/core/src/design.mjs
@@ -1,5 +1,5 @@
-import Pattern from './pattern'
-import { addPartConfig } from './utils.js'
+import Pattern from './pattern.mjs'
+import { addPartConfig } from './utils.mjs'
 
 /*
  * The Design constructor. Returns a Pattern constructor
diff --git a/packages/core/src/hooks.js b/packages/core/src/hooks.mjs
similarity index 100%
rename from packages/core/src/hooks.js
rename to packages/core/src/hooks.mjs
diff --git a/packages/core/src/index.js b/packages/core/src/index.mjs
similarity index 74%
rename from packages/core/src/index.js
rename to packages/core/src/index.mjs
index 56d84cc0f30..7b595ae5359 100644
--- a/packages/core/src/index.js
+++ b/packages/core/src/index.mjs
@@ -1,9 +1,9 @@
-import Design from './design'
-import Pattern from './pattern'
-import Point from './point'
-import Path from './path'
-import Snippet from './snippet'
-import * as utils from './utils'
+import Design from './design.mjs'
+import Pattern from './pattern.mjs'
+import Point from './point.mjs'
+import Path from './path.mjs'
+import Snippet from './snippet.mjs'
+import * as utils from './utils.mjs'
 import pkg from '../package.json' assert { type: 'json' }
 
 const { version } = pkg
diff --git a/packages/core/src/option.js b/packages/core/src/option.mjs
similarity index 100%
rename from packages/core/src/option.js
rename to packages/core/src/option.mjs
diff --git a/packages/core/src/part.js b/packages/core/src/part.mjs
similarity index 98%
rename from packages/core/src/part.js
rename to packages/core/src/part.mjs
index 463bb510f21..78265d05d88 100644
--- a/packages/core/src/part.js
+++ b/packages/core/src/part.mjs
@@ -1,9 +1,9 @@
-import * as utils from './utils'
-import Point from './point'
-import Path from './path'
-import Snippet from './snippet'
-import Attributes from './attributes'
-import Hooks from './hooks'
+import * as utils from './utils.mjs'
+import Point from './point.mjs'
+import Path from './path.mjs'
+import Snippet from './snippet.mjs'
+import Attributes from './attributes.mjs'
+import Hooks from './hooks.mjs'
 
 function Part() {
   this.attributes = new Attributes()
diff --git a/packages/core/src/path.js b/packages/core/src/path.mjs
similarity index 99%
rename from packages/core/src/path.js
rename to packages/core/src/path.mjs
index 3ae6b21ac91..bb103e5fc41 100644
--- a/packages/core/src/path.js
+++ b/packages/core/src/path.mjs
@@ -1,5 +1,5 @@
-import Attributes from './attributes'
-import Point from './point'
+import Attributes from './attributes.mjs'
+import Point from './point.mjs'
 import { Bezier } from 'bezier-js'
 import {
   linesIntersect,
@@ -9,7 +9,7 @@ import {
   pointOnCurve,
   curveEdge,
   round,
-} from './utils'
+} from './utils.mjs'
 
 function Path(debug = false) {
   this.render = true
diff --git a/packages/core/src/pattern.js b/packages/core/src/pattern.mjs
similarity index 98%
rename from packages/core/src/pattern.js
rename to packages/core/src/pattern.mjs
index aa5d8827592..32b296b3b65 100644
--- a/packages/core/src/pattern.js
+++ b/packages/core/src/pattern.mjs
@@ -1,3 +1,4 @@
+import pack from 'bin-pack'
 import {
   macroName,
   sampleStyle,
@@ -5,16 +6,15 @@ import {
   decoratePartDependency,
   addPartConfig,
   mergeDependencies,
-} from './utils.js'
-import Part from './part'
-import Point from './point'
-import Path from './path'
-import Snippet from './snippet'
-import Svg from './svg'
-import pack from 'bin-pack'
-import Store from './store'
-import Hooks from './hooks'
-import Attributes from './attributes'
+} from './utils.mjs'
+import Part from './part.mjs'
+import Point from './point.mjs'
+import Path from './path.mjs'
+import Snippet from './snippet.mjs'
+import Svg from './svg.mjs'
+import Store from './store.mjs'
+import Hooks from './hooks.mjs'
+import Attributes from './attributes.mjs'
 import pkg from '../package.json'
 
 export default function Pattern(config = { options: {} }) {
diff --git a/packages/core/src/point.js b/packages/core/src/point.mjs
similarity index 99%
rename from packages/core/src/point.js
rename to packages/core/src/point.mjs
index 7a138962bcf..9c50715d5f8 100644
--- a/packages/core/src/point.js
+++ b/packages/core/src/point.mjs
@@ -1,4 +1,4 @@
-import Attributes from './attributes'
+import Attributes from './attributes.mjs'
 
 function Point(x, y, debug = false) {
   this.x = x
diff --git a/packages/core/src/snippet.js b/packages/core/src/snippet.mjs
similarity index 95%
rename from packages/core/src/snippet.js
rename to packages/core/src/snippet.mjs
index f6bc08549a8..5b796c57554 100644
--- a/packages/core/src/snippet.js
+++ b/packages/core/src/snippet.mjs
@@ -1,4 +1,4 @@
-import Attributes from './attributes'
+import Attributes from './attributes.mjs'
 
 function Snippet(def, anchor, debug = false) {
   this.def = def
diff --git a/packages/core/src/splitcurve.js b/packages/core/src/splitcurve.mjs
similarity index 100%
rename from packages/core/src/splitcurve.js
rename to packages/core/src/splitcurve.mjs
diff --git a/packages/core/src/store.js b/packages/core/src/store.mjs
similarity index 100%
rename from packages/core/src/store.js
rename to packages/core/src/store.mjs
diff --git a/packages/core/src/svg.js b/packages/core/src/svg.mjs
similarity index 99%
rename from packages/core/src/svg.js
rename to packages/core/src/svg.mjs
index 69f07f0b1d1..c607983a751 100644
--- a/packages/core/src/svg.js
+++ b/packages/core/src/svg.mjs
@@ -1,5 +1,5 @@
-import Attributes from './attributes'
-import { round } from './utils'
+import Attributes from './attributes.mjs'
+import { round } from './utils.mjs'
 import pkg from '../package.json'
 
 function Svg(pattern) {
diff --git a/packages/core/src/utils.js b/packages/core/src/utils.mjs
similarity index 99%
rename from packages/core/src/utils.js
rename to packages/core/src/utils.mjs
index 6b3687d4969..17a7cd0c101 100644
--- a/packages/core/src/utils.js
+++ b/packages/core/src/utils.mjs
@@ -1,6 +1,6 @@
-import Path from './path'
-import Point from './point'
 import { Bezier } from 'bezier-js'
+import Path from './path.mjs'
+import Point from './point.mjs'
 
 export function capitalize(string) {
   return string.charAt(0).toUpperCase() + string.slice(1)