Add type assertions for callback parameters

src/contract_wrappers/exchange_wrapper.ts

@@ -630,6 +630,7 @@ export class ExchangeWrapper extends ContractWrapper {
                                 callback: EventCallback): Promise<string> {
         assert.doesBelongToStringEnum('eventName', eventName, ExchangeEvents);
         assert.doesConformToSchema('indexFilterValues', indexFilterValues, schemas.indexFilterValuesSchema);
+        assert.isFunction('callback', callback);
         const exchangeContractAddress = await this.getContractAddressAsync();
         const subscriptionToken = this._subscribe(
             exchangeContractAddress, eventName, indexFilterValues, artifacts.ExchangeArtifact.abi, callback,

src/contract_wrappers/token_wrapper.ts

@@ -258,6 +258,7 @@ export class TokenWrapper extends ContractWrapper {
         assert.isETHAddressHex('tokenAddress', tokenAddress);
         assert.doesBelongToStringEnum('eventName', eventName, TokenEvents);
         assert.doesConformToSchema('indexFilterValues', indexFilterValues, schemas.indexFilterValuesSchema);
+        assert.isFunction('callback', callback);
         const subscriptionToken = this._subscribe(
             tokenAddress, eventName, indexFilterValues, artifacts.TokenArtifact.abi, callback,
         );

src/utils/assert.ts

@@ -17,6 +17,9 @@ export const assert = {
     isString(variableName: string, value: string): void {
         this.assert(_.isString(value), this.typeAssertionMessage(variableName, 'string', value));
     },
+    isFunction(variableName: string, value: any): void {
+        this.assert(_.isFunction(value), this.typeAssertionMessage(variableName, 'function', value));
+    },
     isHexString(variableName: string, value: string): void {
         this.assert(_.isString(value) && HEX_REGEX.test(value),
             this.typeAssertionMessage(variableName, 'HexString', value));