Add getLinkToken (#96)

Function getLinkToken added #95

Author: SombreroElGringo

docs/api-reference/client.md

@@ -43,6 +43,9 @@ class Client {
   setDefaultRichMenu(richMenuId: string): Promise<{}>
   getDefaultRichMenuId(): Promise<string>
   deleteDefaultRichMenu(): Promise<{}>
+
+  // Account link
+  getLinkToken(userId: string): Promise<string>
 }
 ```
 
@@ -354,3 +357,13 @@ It corresponds to the [Get default rich menu ID](https://developers.line.me/en/r
 ### `deleteDefaultRichMenu(): Promise<{}>`
 
 It corresponds to the [Cancel default rich menu](https://developers.line.me/en/reference/messaging-api/#cancel-default-rich-menu) API.
+
+### Account link
+
+#### `getLinkToken(userId: string): Promise<string>`
+
+Send an HTTP POST request to the `/bot/user/{userId}/linkToken` endpoint, 
+and [issue a link token](https://developers.line.me/en/reference/messaging-api/#issue-link-token) for the user you are attempting to link.
+
+If the request succeeds, a link token will be returned. 
+Link tokens are valid for 10 minutes and can only be used once.

lib/client.ts

@@ -200,4 +200,11 @@ export default class Client {
   public deleteDefaultRichMenu(): Promise<{}> {
     return this.http.delete("/user/all/richmenu");
   }
+
+  public getLinkToken(userId: string): Promise<string> {
+    return this.http
+      .post<any>(`/user/${userId}/linkToken`)
+      .then(checkJSON)
+      .then(res => res.linkToken);
+  }
 }

test/client.spec.ts

@@ -323,4 +323,13 @@ describe("client", () => {
       deepEqual(res, {});
     });
   });
+
+  it("getLinkToken", () => {
+    return client.getLinkToken("test_user_id").then(() => {
+      const req = getRecentReq();
+      equal(req.headers.authorization, "Bearer test_channel_access_token");
+      equal(req.path, "/user/test_user_id/linkToken");
+      equal(req.method, "POST");
+    });
+  });
 });