diff --git a/packages/util/src/aes.js b/packages/util/src/aes.ts
similarity index 100%
rename from packages/util/src/aes.js
rename to packages/util/src/aes.ts
diff --git a/packages/util/src/array.js b/packages/util/src/array.ts
similarity index 100%
rename from packages/util/src/array.js
rename to packages/util/src/array.ts
diff --git a/packages/util/src/index.js b/packages/util/src/index.ts
similarity index 100%
rename from packages/util/src/index.js
rename to packages/util/src/index.ts
diff --git a/packages/util/src/object.js b/packages/util/src/object.ts
similarity index 100%
rename from packages/util/src/object.js
rename to packages/util/src/object.ts
diff --git a/packages/util/src/osm.js b/packages/util/src/osm.ts
similarity index 93%
rename from packages/util/src/osm.js
rename to packages/util/src/osm.ts
index 6b75b5d..d0b027a 100644
--- a/packages/util/src/osm.js
+++ b/packages/util/src/osm.ts
@@ -59,10 +59,10 @@ export function utilCleanTags(tags) {
 //   service: [ 'driveway', 'parking_aisle' ],
 //   width: [ '3', undefined ]
 // }
-export function utilCombinedTags(entityIDs, graph) {
+export function utilCombinedTags(entityIDs: [string], graph) {
   let tags = {};
   let tagCounts = {};
-  let allKeys = new Set();
+  let allKeys: Set<string> = new Set();
 
   const entities = entityIDs.map((entityID) => graph.hasEntity(entityID)).filter(Boolean);
 
@@ -100,14 +100,14 @@ export function utilCombinedTags(entityIDs, graph) {
     });
   });
 
-  for (let key in tags) {
-    if (!Array.isArray(tags[key])) continue;
+  for (let k in tags) {
+    if (!Array.isArray(tags[k])) continue;
 
     // sort values by frequency then alphabetically
-    tags[key] = tags[key].sort((val1, val2) => {
-      let key = key; // capture
-      let count2 = tagCounts[key + '=' + val2];
-      let count1 = tagCounts[key + '=' + val1];
+    tags[k] = tags[k].sort((val1, val2) => {
+      const key = k;
+      const count2 = tagCounts[key + '=' + val2];
+      const count1 = tagCounts[key + '=' + val1];
       if (count2 !== count1) return count2 - count1;
       if (val2 && val1) return val1.localeCompare(val2);
       return val1 ? 1 : -1;
@@ -233,9 +233,17 @@ export function utilGetAllNodes(ids, graph) {
 //
 //
 //
+type TagDiff = {
+  type: string,
+  key: string,
+  oldVal: any,
+  newVal: any,
+  display: string
+};
+
 export function utilTagDiff(oldTags, newTags) {
-  let tagDiff = [];
-  const keys = utilArrayUnion(Object.keys(oldTags), Object.keys(newTags)).sort();
+  let tagDiff: TagDiff[] = [];
+  const keys = utilArrayUnion(Object.keys(oldTags), Object.keys(newTags)).sort() as [string];
   keys.forEach((k) => {
     const oldVal = oldTags[k];
     const newVal = newTags[k];
diff --git a/packages/util/src/session_mutex.js b/packages/util/src/session_mutex.js
deleted file mode 100644
index 2dd3045..0000000
--- a/packages/util/src/session_mutex.js
+++ /dev/null
@@ -1,38 +0,0 @@
-// A per-domain session mutex backed by a cookie and dead man's
-// switch. If the session crashes, the mutex will auto-release
-// after 5 seconds.
-
-// This accepts a string and returns an object that complies with utilSessionMutexType
-export function utilSessionMutex(name) {
-  let _mutex = {};
-  let _intervalID;
-
-  function renew() {
-    let expires = new Date();
-    expires.setSeconds(expires.getSeconds() + 5);
-    document.cookie = name + '=1; expires=' + expires.toUTCString() + '; sameSite=strict';
-  }
-
-  _mutex.lock = () => {
-    if (_intervalID) return true;
-    let cookie = document.cookie.replace(
-      new RegExp('(?:(?:^|.*;)\\s*' + name + '\\s*\\=\\s*([^;]*).*$)|^.*$'),
-      '$1'
-    );
-    if (cookie) return false;
-    renew();
-    _intervalID = window.setInterval(renew, 4000);
-    return true;
-  };
-
-  _mutex.unlock = () => {
-    if (!_intervalID) return;
-    document.cookie = name + '=; expires=Thu, 01 Jan 1970 00:00:00 GMT; sameSite=strict';
-    clearInterval(_intervalID);
-    _intervalID = null;
-  };
-
-  _mutex.locked = () => !!_intervalID;
-
-  return _mutex;
-}
diff --git a/packages/util/src/session_mutex.ts b/packages/util/src/session_mutex.ts
new file mode 100644
index 0000000..c53ad16
--- /dev/null
+++ b/packages/util/src/session_mutex.ts
@@ -0,0 +1,38 @@
+// A per-domain session mutex backed by a cookie and dead man's
+// switch. If the session crashes, the mutex will auto-release
+// after 5 seconds.
+
+// This accepts a string and returns an object that complies with utilSessionMutexType
+export function utilSessionMutex(name) {
+  // let _mutex = {};
+  let _intervalID;
+
+  function renew() {
+    let expires = new Date();
+    expires.setSeconds(expires.getSeconds() + 5);
+    document.cookie = name + '=1; expires=' + expires.toUTCString() + '; sameSite=strict';
+  }
+
+  let _mutex = {
+    lock: () => {
+      if (_intervalID) return true;
+      let cookie = document.cookie.replace(
+        new RegExp('(?:(?:^|.*;)\\s*' + name + '\\s*\\=\\s*([^;]*).*$)|^.*$'),
+        '$1'
+      );
+      if (cookie) return false;
+      renew();
+      _intervalID = window.setInterval(renew, 4000);
+      return true;
+    },
+    unlock: () => {
+      if (!_intervalID) return;
+      document.cookie = name + '=; expires=Thu, 01 Jan 1970 00:00:00 GMT; sameSite=strict';
+      clearInterval(_intervalID);
+      _intervalID = null;
+    },
+    locked: () => { return !!_intervalID; }
+  };
+
+  return _mutex;
+}
diff --git a/packages/util/src/string.js b/packages/util/src/string.ts
similarity index 94%
rename from packages/util/src/string.js
rename to packages/util/src/string.ts
index 658fadd..4dc0ecd 100644
--- a/packages/util/src/string.js
+++ b/packages/util/src/string.ts
@@ -9,16 +9,15 @@ export function utilEditDistance(a, b) {
   if (a.length === 0) return b.length;
   if (b.length === 0) return a.length;
 
-  let matrix = [];
-  let i, j;
-  for (i = 0; i <= b.length; i++) {
+  let matrix: number[][] = [];
+  for (let i = 0; i <= b.length; i++) {
     matrix[i] = [i];
   }
-  for (j = 0; j <= a.length; j++) {
+  for (let j = 0; j <= a.length; j++) {
     matrix[0][j] = j;
   }
-  for (i = 1; i <= b.length; i++) {
-    for (j = 1; j <= a.length; j++) {
+  for (let i = 1; i <= b.length; i++) {
+    for (let j = 1; j <= a.length; j++) {
       if (b.charAt(i - 1) === a.charAt(j - 1)) {
         matrix[i][j] = matrix[i - 1][j - 1];
       } else {
diff --git a/packages/util/tests/aes.test.js b/packages/util/tests/aes.test.ts
similarity index 100%
rename from packages/util/tests/aes.test.js
rename to packages/util/tests/aes.test.ts
diff --git a/packages/util/tests/array.test.js b/packages/util/tests/array.test.ts
similarity index 100%
rename from packages/util/tests/array.test.js
rename to packages/util/tests/array.test.ts
diff --git a/packages/util/tests/object.test.js b/packages/util/tests/object.test.ts
similarity index 100%
rename from packages/util/tests/object.test.js
rename to packages/util/tests/object.test.ts
diff --git a/packages/util/tests/osm.test.js b/packages/util/tests/osm.test.ts
similarity index 100%
rename from packages/util/tests/osm.test.js
rename to packages/util/tests/osm.test.ts
diff --git a/packages/util/tests/session_mutex.test.js b/packages/util/tests/session_mutex.test.ts
similarity index 64%
rename from packages/util/tests/session_mutex.test.js
rename to packages/util/tests/session_mutex.test.ts
index 26b675f..04766d3 100644
--- a/packages/util/tests/session_mutex.test.js
+++ b/packages/util/tests/session_mutex.test.ts
@@ -5,48 +5,61 @@ import * as util from '../src/index';
 describe('utilSessionMutex', () => {
   let a, b;
 
+  // Hack: Rename methods so esbuild-jest doesn't mistransform them...
+  function esbuildJestHack(obj) {
+    obj.lok = obj.lock;
+    obj.unlok = obj.unlock;
+  }
+
   afterEach(() => {
-    if (a) a.unlock();
-    if (b) b.unlock();
+    if (a) a.unlok();
+    if (b) b.unlok();
   });
 
   describe('#lock', () => {
     it('returns true when it gets a lock', () => {
       a = util.utilSessionMutex('name');
-      expect(a.lock()).toBeTrue();
+      esbuildJestHack(a);
+      expect(a.lok()).toBeTrue();
     });
 
     it('returns true when already locked', () => {
       a = util.utilSessionMutex('name');
-      a.lock();
-      expect(a.lock()).toBeTrue();
+      esbuildJestHack(a);
+      a.lok();
+      expect(a.lok()).toBeTrue();
     });
 
     it('returns false when the lock is held by another session', () => {
       a = util.utilSessionMutex('name');
-      a.lock();
+      esbuildJestHack(a);
+      a.lok();
 
       b = util.utilSessionMutex('name');
-      expect(b.lock()).toBeFalse();
+      esbuildJestHack(b);
+      expect(b.lok()).toBeFalse();
     });
   });
 
   describe('#locked', () => {
     it('returns false by default', () => {
       a = util.utilSessionMutex('name');
+      esbuildJestHack(a);
       expect(a.locked()).toBeFalse();
     });
 
     it('returns true when locked', () => {
       a = util.utilSessionMutex('name');
-      a.lock();
+      esbuildJestHack(a);
+      a.lok();
       expect(a.locked()).toBeTrue();
     });
 
     it('returns false when unlocked', () => {
       a = util.utilSessionMutex('name');
-      a.lock();
-      a.unlock();
+      esbuildJestHack(a);
+      a.lok();
+      a.unlok();
       expect(a.locked()).toBeFalse();
     });
   });
@@ -54,36 +67,43 @@ describe('utilSessionMutex', () => {
   describe('#unlock', () => {
     it('unlocks the mutex', () => {
       a = util.utilSessionMutex('name');
-      a.lock();
-      a.unlock();
+      esbuildJestHack(a);
+      a.lok();
+      a.unlok();
 
       b = util.utilSessionMutex('name');
-      expect(b.lock()).toBeTrue();
+      esbuildJestHack(b);
+      expect(b.lok()).toBeTrue();
     });
 
     it('does nothing when the lock is held by another session', () => {
       a = util.utilSessionMutex('name');
-      a.lock();
+      esbuildJestHack(a);
+      a.lok();
 
       b = util.utilSessionMutex('name');
-      b.unlock();
+      esbuildJestHack(b);
+      b.unlok();
 
       expect(a.locked()).toBeTrue();
     });
 
     it('does nothing when not locked', () => {
       a = util.utilSessionMutex('name');
-      a.unlock();
+      esbuildJestHack(a);
+      a.unlok();
       expect(a.locked()).toBeFalse();
     });
   });
 
   it('namespaces locks', () => {
     a = util.utilSessionMutex('a');
-    a.lock();
+    esbuildJestHack(a);
+    a.lok();
 
     b = util.utilSessionMutex('b');
+    esbuildJestHack(b);
     expect(b.locked()).toBeFalse();
-    expect(b.lock()).toBeTrue();
+    expect(b.lok()).toBeTrue();
   });
 });
diff --git a/packages/util/tests/string.test.js b/packages/util/tests/string.test.ts
similarity index 100%
rename from packages/util/tests/string.test.js
rename to packages/util/tests/string.test.ts