Atomically sets key to value and returns the value previously stored at key.
|key||string||the key to get and set|
|value||string, number, or boolean||the value to set|
|Type||Resolves with||Rejected when|
|Promise<string>||On success, the promise resolves with the old value stored at key.||If key does not exist, or does not hold a string value, the promise is rejected with an error.|