Implement ERC20Wrapper and it's first method getBalanceAsync

src/0x.js.ts

@@ -13,6 +13,7 @@ import compareVersions = require('compare-versions');
 import {ExchangeWrapper} from './contract_wrappers/exchange_wrapper';
 import {TokenRegistryWrapper} from './contract_wrappers/token_registry_wrapper';
 import {ecSignatureSchema} from './schemas/ec_signature_schema';
+import {ERC20Wrapper} from './contract_wrappers/erc20_wrapper';
 import {SolidityTypes, ECSignature, ZeroExError} from './types';
 
 const MAX_DIGITS_IN_UNSIGNED_256_INT = 78;
@@ -21,6 +22,7 @@ export class ZeroEx {
     public web3Wrapper: Web3Wrapper;
     public exchange: ExchangeWrapper;
     public tokenRegistry: TokenRegistryWrapper;
+    public erc20: ERC20Wrapper;
     /**
      * Computes the orderHash given the order parameters and returns it as a hex encoded string.
      */
@@ -135,6 +137,7 @@ export class ZeroEx {
         this.web3Wrapper = new Web3Wrapper(web3);
         this.exchange = new ExchangeWrapper(this.web3Wrapper);
         this.tokenRegistry = new TokenRegistryWrapper(this.web3Wrapper);
+        this.erc20 = new ERC20Wrapper(this.web3Wrapper);
     }
     /**
      * Signs an orderHash and returns it's elliptic curve signature

src/contract_wrappers/erc20_wrapper.ts

@@ -0,0 +1,28 @@
+import * as _ from 'lodash';
+import * as BigNumber from 'bignumber.js';
+import {Web3Wrapper} from '../web3_wrapper';
+import {assert} from '../utils/assert';
+import {ContractWrapper} from './contract_wrapper';
+import * as TokenArtifacts from '../artifacts/Token.json';
+import {ERC20Contract} from '../types';
+
+export class ERC20Wrapper extends ContractWrapper {
+    constructor(web3Wrapper: Web3Wrapper) {
+        super(web3Wrapper);
+    }
+    /**
+     * Returns an owner's ERC20 token balance
+     */
+    public async getBalanceAsync(tokenAddress: string, ownerAddress: string): Promise<BigNumber.BigNumber> {
+        assert.isETHAddressHex('ownerAddress', ownerAddress);
+        assert.isETHAddressHex('tokenAddress', tokenAddress);
+
+        const contractInstance = await this.instantiateContractIfExistsAsync((TokenArtifacts as any), tokenAddress);
+        const tokenContract = contractInstance as ERC20Contract;
+        let balance = await tokenContract.balanceOf.call(ownerAddress);
+        // The BigNumber instance returned by Web3 is of a much older version then our own, we therefore
+        // should always re-instantiate the returned BigNumber after retrieval.
+        balance = _.isUndefined(balance) ? new BigNumber(0) : new BigNumber(balance);
+        return balance;
+    }
+}

src/types.ts

@@ -30,6 +30,12 @@ export interface ExchangeContract {
     isValidSignature: any;
 }
 
+export interface ERC20Contract {
+    balanceOf: {
+        call: (address: string) => Promise<BigNumber.BigNumber>;
+    };
+}
+
 export interface TokenRegistryContract {
     getTokenMetaData: any;
     getTokenAddresses: any;