Albert Barnes Commentary 2 Kings 11:12

"Then he brought out the king`s son, and put the crown upon him, and [gave him] the testimony; and they made him king, and anointed him; and they clapped their hands, and said, [Long] live the king." — 2 Kings 11:12 (ASV)

The testimony - that is, “The Book of the Law” which was kept in the ark of the covenant (Deuteronomy 31:26). This Jehoiada placed on the king’s head at the moment of coronation, perhaps to indicate that the king was not to be above, but under, the direction of the Law of his country.