Add getTimestamp() method

__tests__/ain.test.ts

@@ -269,7 +269,12 @@ describe('ain-js', function() {
 
     it('getNonce', async function() {
       const nonce = await ain.wallet.getNonce();
-      expect(nonce).toBeNull();
+      expect(nonce).toBe(0);
+    });
+
+    it('getTimestamp', async function() {
+      const timestamp = await ain.wallet.getTimestamp();
+      expect(timestamp).toBe(0);
     });
 
     it('transfer with isDryrun = true', async function() {

src/signer/default-signer.ts

@@ -131,3 +131,4 @@ export class DefaultSigner implements Signer {
     const billing = transactionInput.billing;
     return Object.assign(tx, { nonce, timestamp, gas_price: gasPrice, billing });
   }
+  };

src/wallet.ts

@@ -268,6 +268,26 @@ export default class Wallet {
     return this.ain.provider.send('ain_getNonce', { address, from: args.from })
   }
 
+  /**
+   * Fetches an account's timestamp value, which is the current transaction count of the account.
+   * @param {object} args The ferch options.
+   * It may contain a string 'address' value and a string 'from' value.
+   * The 'address' is the address of the account to get the timestamp of,
+   * and the 'from' is the source of the data.
+   * It could be either the pending transaction pool ("pending") or
+   * the committed blocks ("committed"). The default value is "committed".
+   * @returns {Promise<number>} The timestamp value.
+   */
+  getTimestamp(args: { address?: string, from?: string }): Promise<number> {
+    if (!args) { args = {}; }
+    const address = args.address ? Ain.utils.toChecksumAddress(args.address)
+      : this.getImpliedAddress(args.address);
+    if (args.from !== undefined && args.from !== 'pending' && args.from !== 'committed') {
+      throw Error("'from' should be either 'pending' or 'committed'");
+    }
+    return this.ain.provider.send('ain_getTimestamp', { address, from: args.from })
+  }
+
   /**
    * Sends a transfer transaction to the network.
    * @param {{to: string, value: number, from?: string, nonce?: number, gas_price?: number}} input The input parameters of the transaction.