diff --git a/js/igv.d.ts b/js/igv.d.ts
index db909dff9..0eb015065 100644
--- a/js/igv.d.ts
+++ b/js/igv.d.ts
@@ -4,10 +4,12 @@ type AnyCase<T extends string> = Uppercase<T> | Lowercase<T>;
 // any action that can be deferred as a promise or a thenable
 type Deferrable<T> = T | Promise<T> | Thenable<T, any> | (() => T | Promise<T> | Thenable<T, any>);
 
+// a custom future implementation supported by igv.js
 export interface Thenable<T, E> {
     then(resolve: (value: T) => void, reject: (error: E) => void): void;
 }
 
+// simply prevents the type from being inferred
 declare class Opaque<N extends string> {
     private readonly __opaque_brand: N;
 }
@@ -18,6 +20,7 @@ export type TrackType =
     /* undocumented options */
     "snp" | "eqtl";
 
+// converts a track type into the options needed to create it
 export type TrackLoad<T extends TrackType> =
     Tracks.TrackCommonOptions &
     (T extends "annotation" ? Tracks.AnnotationTrackOptions & TypeFormatPair<'annotation'> :
@@ -40,23 +43,27 @@ export type TrackLoad<T extends TrackType> =
 
 export type TypeFormatPair<T extends TrackType> = {
     type?: T;
+    // if we know the format, type is not required
     format: TrackFormatOf<T>;
     url?: string;
 } | {
     type?: T;
     format?: TrackFormatOf<T>;
+    // if we can infer the URL, we do not need type or format
     url: TrackFormatOf<T> extends string ? Deferrable<URLInference.URLWithExtension<TrackFormatOf<T>>> : never;
 } | ({
-    type?: T;
+    // an explicit type and manual reader is allowed, however format and url will not make sense in this context
+    type: T;
     format?: never;
     url?: never;
 } & (CustomReaderOf<T> | ManualFeatureOf<T>)) | {
     type: T;
     format?: TrackFormatOf<T>;
-    // if there is only one possible format, we permit it to be omitted
-    url: TrackFormatOf<T> extends string ? (string extends TrackFormatOf<T> ? never : string) : never;
+    // if there is only one possible format, we permit format to be omitted
+    url: TrackFormatOf<T> extends string ? (string extends TrackFormatOf<T> ? never : Deferrable<URLInference.URLWithExtension<TrackFormatOf<T>>>) : never;
 };
 
+// converts a track type into the file formats that it can accept
 export type TrackFormatOf<T extends TrackType> =
     T extends "annotation" ? Tracks.AnnotationFormat :
     T extends "wig" ? Tracks.WigFormat :
@@ -75,6 +82,7 @@ export type TrackFormatOf<T extends TrackType> =
     T extends "eqtl" ? Tracks.EQTLFormat :
     never;
 
+// converts a track type into the manual features that it can accept
 export type ManualFeatureOf<T extends TrackType> =
     T extends "annotation" ? {
         features: Record<string, any>[];
@@ -83,15 +91,16 @@ export type ManualFeatureOf<T extends TrackType> =
         features: Tracks.WigTrackManualFeatures;
     } :
     T extends "seg" ? {
-        features: Array<{
+        features: {
             chr: string;
             start: number;
             end: number;
             value: number;
             sample: string;
-        }>;
+        }[];
     } : never;
 
+// some tracks can support a custom reader that polls data based on options
 export type CustomReaderOf<T extends TrackType> =
     T extends "annotation" ? { reader: Tracks.AnnotationCustomReader } |
     {
@@ -113,6 +122,7 @@ export type CustomReaderOf<T extends TrackType> =
     } :
     never;
 
+// converts a track type into the actual track class
 export type TrackOf<T extends TrackType> =
     T extends "annotation" ? Tracks.Track :
     T extends "wig" ? Tracks.Track :
@@ -151,10 +161,12 @@ export namespace URLInference {
 
     type AuxExtensions = ".gz" | ".tab" | ".txt" | ".bgz";
 
+    // infer the extension of a URL, with auxillary extensions stripped
     export type InferExtension<F extends string> = Lowercase<F> extends `${infer B}${AuxExtensions}` ? StripLast<B, '.'> : StripLast<Lowercase<F>, '.'>
 
     type Query = `?${string}`;
 
+    // matches a URL with an extension
     export type URLWithExtension<E extends string> =
         `${string}.${E}${AuxExtensions | ''}${Query | ''}`;
 }
@@ -370,7 +382,7 @@ export namespace Tracks {
          * 
          * @type {object}
          */
-        guidelines?: Array<{ color: string, y: number, dotted: boolean }>;
+        guidelines?: { color: string, y: number, dotted: boolean }[];
         /**
          * Type of graph, either "bar" or "points"
          * 
@@ -634,8 +646,6 @@ export namespace Tracks {
 }
 
 type Nucleotide = 'A' | 'C' | 'G' | 'T' | 'N';
-type AnnotationFormat = 'bed' | 'gff3' | 'gtf' | 'genePred' | 'genePredExt' | 'peaks' | 'narrowPeak' | 'broadPeak' | 'bigBed' | 'bedpe';
-
 
 type HostedGenomes = "hs1" | "chm13v1.1" | "hg38" | "hg38_1kg" | "hg19" |
     "hg18" | "mm39" | "mm10" | "mm9" | "rn7" |
@@ -788,6 +798,7 @@ interface CreateOptExtras {
 export namespace BrowserEvents {
     export type EventType = "trackremoved" | "trackdrag" | "trackdragend" | "locuschange" | "trackclick" | "trackorderchanged";
 
+    // returns the type of the event handler based on the event type
     export type EventHandler<T extends EventType> =
         T extends "trackremoved" ? (tracks: Tracks.Track[]) => EventReturn<T> :
         T extends "locusChange" ? (loci: {
@@ -809,6 +820,7 @@ export namespace BrowserEvents {
 }
 
 export type CreateOpt = (GenomeOpt & CreateOptExtras) | (
+    // if a session URL is provided, we do not need a complete definition, additionally we should not allow a different genome
     {
         sessionURL: string;
     } & Partial<CreateOptExtras>