How did the Americans learn about the Japanese plan to attack on Midway Island?

After a sudden multiple attack on Aleutian Islands and off the coast of Alaska, Japanese planned a prolonged attack towards Midway.

Explanation:

Japan's aim was to prove its superiority over naval and air force of America firstly it attacked Aleutian islands and off the coast of Alaska. Thereafter they planned for a prolonged attack on America. But from their previous sudden attack America got to know about their future plan.

U.S navy defeated Japan's sudden attack and won the battle of midway. To protect women's role it was important to America to combat Japan and defeat them in the midway battle.