diff --git a/js/examples/maplibre.html b/js/examples/maplibre.html
index 80e89df..1ada9e9 100644
--- a/js/examples/maplibre.html
+++ b/js/examples/maplibre.html
@@ -23,7 +23,7 @@
 
             let PMTILES_URL = "https://protomaps.github.io/PMTiles/protomaps(vector)ODbL_firenze.pmtiles";
 
-            const p = new pmtiles.PMTiles(PMTILES_URL)
+            const p = new pmtiles.PMTiles(PMTILES_URL);
 
             // this is so we share one instance across the JS code and the map renderer
             protocol.add(p);
diff --git a/js/index.ts b/js/index.ts
index f97a0c4..d781e40 100644
--- a/js/index.ts
+++ b/js/index.ts
@@ -282,15 +282,21 @@ export class FileAPISource implements Source {
 
 export class FetchSource implements Source {
   url: string;
+  customHeaders: Headers;
 
-  constructor(url: string) {
+  constructor(url: string, customHeaders: Headers = new Headers()) {
     this.url = url;
+    this.customHeaders = customHeaders;
   }
 
   getKey() {
     return this.url;
   }
 
+  setHeaders(customHeaders: Headers) {
+    this.customHeaders = customHeaders;
+  }
+
   async getBytes(
     offset: number,
     length: number,
@@ -303,9 +309,15 @@ export class FetchSource implements Source {
       signal = controller.signal;
     }
 
+    const requestHeaders = new Headers(this.customHeaders);
+    requestHeaders.set(
+      "Range",
+      "bytes=" + offset + "-" + (offset + length - 1)
+    );
+
     let resp = await fetch(this.url, {
       signal: signal,
-      headers: { Range: "bytes=" + offset + "-" + (offset + length - 1) },
+      headers: requestHeaders,
     });
 
     // TODO: can return 416 with offset > 0 if content changed, which will have a blank etag.