exec/execFile result value is in fact string|Buffer

See nodejs/node#3389

node/node-tests.ts

@@ -9,7 +9,7 @@ import * as crypto from "crypto";
 import * as tls from "tls";
 import * as http from "http";
 import * as net from "net";
-import * as dgram from "dgram";
+import * as dgram from "dgram";3
 import * as querystring from "querystring";
 import * as path from "path";
 import * as readline from "readline";
@@ -409,3 +409,40 @@ rl.question("do you like typescript?", function(answer: string) {
 
 childProcess.exec("echo test");
 childProcess.spawnSync("echo test");
+
+//////////////////////////////////////////////////////////////////
+/// Check for exec / execFile result type, in case spec changes///
+/// https://github.com/nodejs/node/issues/3389                 ///
+//////////////////////////////////////////////////////////////////
+
+function execFileTest(opt:Object,cb:(stdout:any)=>void) {
+  childProcess.execFile('/bin/echo', ['test'], opt,
+  (error, stdout, stderr) => {
+    assert(!error);
+    return cb(stdout);
+  });
+}
+
+function execTest(opt:Object,cb:(stdout:any)=>void) {
+  childProcess.exec('/bin/echo', opt,
+  (error, stdout, stderr) => {
+    assert(!error);
+    return cb(stdout);
+  });
+}
+
+[execFileTest,execTest].forEach(test=>{
+  test({}, stdout => {
+    assert.equal(typeof stdout, 'string', 'Default encoding is utf8, should return a string');
+  });
+
+  test({encoding:'utf8'}, stdout => {
+    assert.equal(typeof stdout, 'string', 'Encoding is explicitly set as utf8, should return a string');
+  });
+
+  test({encoding:'buffer'}, stdout => {
+    assert.equal(typeof stdout, 'object', 'Encoding set as "buffer", should return an object');
+    assert.equal(Object.getPrototypeOf(stdout).constructor.name, 'Buffer',
+      'Encoding set as "buffer", result\'s constructor should be named Buffer');
+  });
+})

node/node.d.ts

@@ -843,26 +843,26 @@ declare module "child_process" {
         stdio?: any;
         customFds?: any;
         env?: any;
-        encoding?: string;
+        encoding?: string; /// default: 'utf8'. Use 'buffer' to get a Buffer result instead of a string
         timeout?: number;
         maxBuffer?: number;
         killSignal?: string;
-    }, callback?: (error: Error, stdout: Buffer, stderr: Buffer) =>void ): ChildProcess;
-    export function exec(command: string, callback?: (error: Error, stdout: Buffer, stderr: Buffer) =>void ): ChildProcess;
+    }, callback?: (error: Error, stdout: string|Buffer, stderr: string | Buffer) =>void ): ChildProcess;
+    export function exec(command: string, callback?: (error: Error, stdout: string | Buffer, stderr: string | Buffer) =>void ): ChildProcess;
     export function execFile(file: string,
-        callback?: (error: Error, stdout: Buffer, stderr: Buffer) =>void ): ChildProcess;
+        callback?: (error: Error, stdout: string | Buffer, stderr: string | Buffer) =>void ): ChildProcess;
     export function execFile(file: string, args?: string[],
-        callback?: (error: Error, stdout: Buffer, stderr: Buffer) =>void ): ChildProcess;
+        callback?: (error: Error, stdout: string | Buffer, stderr: string | Buffer) =>void ): ChildProcess;
     export function execFile(file: string, args?: string[], options?: {
         cwd?: string;
         stdio?: any;
         customFds?: any;
         env?: any;
-        encoding?: string;
+        encoding?: string; /// default: 'utf8'. Use 'buffer' to get a Buffer result instead of a string
         timeout?: number;
         maxBuffer?: number;
         killSignal?: string;
-    }, callback?: (error: Error, stdout: Buffer, stderr: Buffer) =>void ): ChildProcess;
+    }, callback?: (error: Error, stdout: string | Buffer, stderr: string | Buffer) =>void ): ChildProcess;
     export function fork(modulePath: string, args?: string[], options?: {
         cwd?: string;
         env?: any;