### Given an api that looks like this:

```javascript
    this.client = redis.createClient()

    const asyncSet = promisify(this.client.set).bind(this.client)

    return asyncSet(key, value, "EX", expiry)
```


### The correct corresponding mock looks like:

```javascript
    jest.mock('redis', () => ({
        createClient: jest.fn(() => ({
            set: jest.fn((a, b, c, d, callback) => callback(null, true))
        }))
    }))
```

## How promisify works:

Given an function that normally takes a callback as the LAST parameter:

```javascript
  function(a,b,c,d,callback){
    //do some async thing

    callback(null, "I'm done!") //success response
    OR
    callback("ERROR'd", null) //error response
  }
```

So the overly simplified "promisify'd" code might look like:

```javascript
   const promise = new Promise()

   function(a,b,c,d,callback){
    //do some async thing

      if(<first callback argument is set>){
          promise.resolve("I'm done!")
      }else{
          promise.reject("ERROR'd")
      }

   }
  return promise
```