+ tickFormat also directly accepts a string parameter

d3/d3.d.ts

@@ -2535,6 +2535,7 @@ declare module d3 {
 
             tickFormat(): (t: any) => string;
             tickFormat(format: (t: any) => string): Axis;
+            tickFormat(format:string): Axis;
         }
 
         export function brush(): Brush<any>;