Add completion function

yargs/yargs-tests.ts

@@ -150,6 +150,33 @@ function command() {
 	.argv;
 }
 
+function completion_sync() {
+	var argv = yargs
+	.completion('completion', (current, argv) => {
+		// 'current' is the current command being completed.
+		// 'argv' is the parsed arguments so far.
+		// simply return an array of completions.
+		return [
+			'foo',
+			'bar'
+		];
+	})
+	.argv;
+}
+
+function completion_async() {
+	var argv = yargs
+		.completion('completion', (current, argv, done) => {
+			setTimeout(function() {
+				done([
+					'apple',
+					'banana'
+				]);
+			}, 500);
+		})
+		.argv;
+}
+
 function Argv$help() {
 	var yargs1 = yargs
 		.usage("$0 -operand1 number -operand2 number -operation [add|subtract]");

yargs/yargs.d.ts

@@ -56,6 +56,11 @@ declare module "yargs" {
 			command(command: string, description: string): Argv;
 			command(command: string, description: string, fn: (args: Argv) => void): Argv;
 
+			completion(cmd: string, fn?: SyncCompletionFunction): Argv;
+			completion(cmd: string, description?: string, fn?: SyncCompletionFunction): Argv;
+			completion(cmd: string, fn?: AsyncCompletionFunction): Argv;
+			completion(cmd: string, description?: string, fn?: AsyncCompletionFunction): Argv;
+
 			example(command: string, description: string): Argv;
 
 			check(func: (argv: any, aliases: { [alias: string]: string }) => any): Argv;
@@ -111,6 +116,9 @@ declare module "yargs" {
 			desc?: any;
 			requiresArg?: any;
 		}
+
+		type SyncCompletionFunction = (current: string, argv: any) => string[];
+		type AsyncCompletionFunction = (current: string, argv: any, done: (completion: string[]) => void) => void;
 	}
 
 	var yargs: yargs.Argv;