National Security Agency. "Navy codebreakers at work at Pearl Harbor." Military History Now, 3 February 2021, https://militaryhistorynow.com/2021/02/03/u-s-navy-codebreakers-americas-cryptanalysis-coup-at-the-battle-of-midway-was-20-years-in-the-making/
The Allies used communications intelligence, or COMINT, in the Pacific. There were a couple Allied fleet radio-intercept units positioned throughout the Pacific that intercepted Japanese radio transmissions. They used codebreaking, analysis, and logic to decode messages and reveal Japanese plans, positions, and navy statuses.
U.S. communications intelligence broke the Japanese navy’s main code and identified the Midway Atoll as a possible Japanese objective on March 13, 1942.
On May 6, the U.S. forces at the Philippines Allied Radio-Intercept Unit surrendered to Japan. The equipment was destroyed before the Japanese could use it, and the cryptanalysts fled to Melbourne.
The United States won the Battle of the Coral Sea on May 7-8. The key information of this battle was supplied by Allied communications intelligence, leading to Japan's first setback of the war.
On May 22, Allied communications intelligence completely confirmed that Midway is a Japanese objective and predicted the attack for June 4th. The Japanese did not know this, so they were still expecting them to ambush U.S. forces. However, Allied COMINT allowed the United States to prepare for the battle, leading to strong U.S. forces at Midway.