diff --git a/types/proj4/index.d.ts b/types/proj4/index.d.ts
index afba494556..d7d4cf1ae5 100644
--- a/types/proj4/index.d.ts
+++ b/types/proj4/index.d.ts
@@ -1,6 +1,7 @@
-// Type definitions for proj4 2.3
+// Type definitions for proj4 2.5
 // Project: https://github.com/proj4js/proj4js
 // Definitions by: Denis Carriere <https://github.com/DenisCarriere>
+//                 BendingBender <https://github.com/BendingBender>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 declare namespace proj4 {
@@ -21,9 +22,9 @@ declare namespace proj4 {
         ep2: number;
     }
 
-    interface Static {
-        forward(coordinates: TemplateCoordinates): number[];
-        inverse(coordinates: TemplateCoordinates): number[];
+    interface Converter {
+        forward<T extends TemplateCoordinates>(coordinates: T): T;
+        inverse<T extends TemplateCoordinates>(coordinates: T): T;
     }
 
     interface InterfaceProjection {
@@ -38,6 +39,39 @@ declare namespace proj4 {
         inverse(coordinates: TemplateCoordinates): number[];
     }
 
+    interface ProjectionDefinition {
+        title: string;
+        projName?: string;
+        ellps?: string;
+        datum?: string;
+        datumName?: string;
+        rf?: number;
+        lat0?: number;
+        lat1?: number;
+        lat2?: number;
+        lat_ts?: number;
+        long0?: number;
+        long1?: number;
+        long2?: number;
+        alpha?: number;
+        longc?: number;
+        x0?: number;
+        y0?: number;
+        k0?: number;
+        a?: number;
+        b?: number;
+        R_A?: true;
+        zone?: number;
+        utmSouth?: true;
+        datum_params?: string | number[];
+        to_meter?: number;
+        units?: string;
+        from_greenwich?: number;
+        datumCode?: string;
+        natGrids?: string;
+        axis?: string;
+    }
+
     const defaultDatum: string;
 
     function Proj(srsCode: any, callback?: any): InterfaceProjection;
@@ -45,23 +79,38 @@ declare namespace proj4 {
     const WGS84: any;
 
     /**
-     * Depecrated v3
+     * @deprecated v3
      */
     function Point(x: number, y: number, z?: number): InterfaceCoordinates;
     function Point(coordinates: TemplateCoordinates | string): InterfaceCoordinates;
 
     function toPoint(array: number[]): InterfaceCoordinates;
 
-    function defs(name: string, projection?: string): any;
-    function defs(name: string[][]): any;
+    function defs(name: string, projection: string | ProjectionDefinition): void;
+    function defs(name: string[][]): undefined[];
+    function defs(name: string): ProjectionDefinition;
 
-    function transform(source: InterfaceProjection, dest: InterfaceProjection, point: TemplateCoordinates): any;
+    function transform(
+        source: InterfaceProjection,
+        dest: InterfaceProjection,
+        point: TemplateCoordinates
+    ): any;
 
     function mgrs(coordinates: number[], accuracy: number): string;
 
     const version: string;
 }
 
-declare function proj4(fromProjection: string, toProjection?: string, coordinates?: proj4.TemplateCoordinates): proj4.Static;
-declare function proj4(fromProjection: string, coordinates: proj4.TemplateCoordinates): number[];
+declare function proj4(fromProjection: string, toProjection?: string): proj4.Converter;
+declare function proj4<T extends proj4.TemplateCoordinates>(
+    toProjection: string,
+    coordinates: T
+): T;
+declare function proj4<T extends proj4.TemplateCoordinates>(
+    fromProjection: string,
+    toProjection: string,
+    coordinates: T
+): T;
+
 export = proj4;
+export as namespace proj4;
diff --git a/types/proj4/proj4-tests.ts b/types/proj4/proj4-tests.ts
index 3d1ab1ab9b..7df09014db 100644
--- a/types/proj4/proj4-tests.ts
+++ b/types/proj4/proj4-tests.ts
@@ -3,31 +3,55 @@ import proj4 = require('proj4');
 ///////////////////////////////////////////
 // Tests data initialisation
 ///////////////////////////////////////////
-const name = 'WGS84';
 const epsg = {
     4269: '+title=NAD83 (long/lat) +proj=longlat +a=6378137.0 +b=6356752.31414036 +ellps=GRS80 +datum=NAD83 +units=degrees',
     4326: '+title=WGS 84 (long/lat) +proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees',
 };
-const point1 = [-71, 41];
-const point2 = {x: 2, y: 5};
-const mgrs = "24XWT783908";
+const firstProjection = epsg[4269];
+const secondProjection = epsg[4326];
+
+const pointArr = [-71, 41];
+const pointObj = { x: 2, y: 5 };
 
 ///////////////////////////////////////////
 // Tests Measurement
 ///////////////////////////////////////////
-proj4(epsg['4269'], epsg['4326'], point1);
-proj4(epsg['4269'], point1);
-proj4(epsg['4269'], epsg['4326']).forward(point2);
-proj4(epsg['4269'], epsg['4326']).inverse(point2);
+proj4(firstProjection, secondProjection, pointArr);
+proj4(firstProjection, pointArr);
+
+// $ExpectType number[]
+proj4(firstProjection, secondProjection, pointArr);
+// $ExpectType { x: number; y: number; }
+proj4(firstProjection, secondProjection, pointObj);
+
+// $ExpectType number[]
+proj4(firstProjection, pointArr);
+// $ExpectType { x: number; y: number; }
+proj4(firstProjection, pointObj);
+
+// $ExpectType Converter
+proj4(firstProjection, secondProjection);
+// $ExpectType Converter
+proj4(firstProjection);
+
+// $ExpectType number[]
+proj4(firstProjection, secondProjection).forward(pointArr);
+// $ExpectType { x: number; y: number; }
+proj4(firstProjection, secondProjection).forward(pointObj);
+// $ExpectType number[]
+proj4(firstProjection, secondProjection).inverse(pointArr);
+// $ExpectType { x: number; y: number; }
+proj4(firstProjection, secondProjection).inverse(pointObj);
 
 ///////////////////////////////////
 // Named Projections
 ///////////////////////////////////
-proj4.defs('WGS84', epsg['4326']);
-proj4.defs([
-  ['EPSG:4326', epsg['4326']],
-  ['EPSG:4269', epsg['4269']]
-]);
+
+// $ExpectType ProjectionDefinition
+proj4.defs('WGS84');
+proj4.defs('WGS84', secondProjection);
+// $ExpectType undefined[]
+proj4.defs([['EPSG:4326', secondProjection], ['EPSG:4269', firstProjection]]);
 proj4.defs('urn:x-ogc:def:crs:EPSG:4326', proj4.defs('EPSG:4326'));
 
 ///////////////////////////////////
diff --git a/types/proj4/tsconfig.json b/types/proj4/tsconfig.json
index a55659d52c..02713475d5 100644
--- a/types/proj4/tsconfig.json
+++ b/types/proj4/tsconfig.json
@@ -6,7 +6,7 @@
         ],
         "noImplicitAny": true,
         "noImplicitThis": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "strictFunctionTypes": true,
         "baseUrl": "../",
         "typeRoots": [
@@ -20,4 +20,4 @@
         "index.d.ts",
         "proj4-tests.ts"
     ]
-}
\ No newline at end of file
+}