In the seventh year of Jehu, Jehoash began to reign, and he reigned forty years in Jerusalem. His mother’s name was Zibiah of Beersheba.
The general account here given of Joash is, 1. That he reigned forty years. As he began his reign when he was very young, he might, in the course of nature, have continued much longer, for he was cut off when he was but forty-seven years old, Kg2 12:1. 2. That he did that which was right as long as Jehoiada lived to instruct him, Kg2 12:2.
Commenting on 2 Kings 12:1-3
In the seventh year of Jehu Jehoash began to reign,.... So that he reigned twenty one or twenty two years contemporary with Jehu's reign, for Jehu reigned twenty eight years: and forty years reigned he in Jerusalem; the same number of years David and Solomon reigned: and his mother's name was Zibiah of Beersheba; a city in the tribe of Simeon, in the extreme part...
All that is recorded of the forty years’ reign of Joash, in addition to the general characteristics of the reign (2Ki 12:1-4), is the repairing of the temple which was effected by him (2Ki 12:5-17), and the purchased retreat of the Syrians from their invasion of Judah (2Ki 12:18 and 2Ki 12:19), and finally his violent death in consequence of a conspiracy formed against him...