Array.reduce

Fold an array into one value — sum, max, or a count object — step by step.

reduce walks the array with an accumulator. Always pass an initial value so empty arrays and object totals behave predictably.

Source


        

        

          
Result

          

        

      

      
Accumulator steps

      
Steps appear here when you run a reduce.

      
Pick a reduce demo.

    


    

      
Important JavaScript

      
const scores = [10, 20, 5, 30];

const total = scores.reduce(function (acc, n) {
  return acc + n;
}, 0);
// steps: 0→10→30→35→65

const max = scores.reduce((acc, n) => (n > acc ? n : acc), scores[0]);

const players = [
  { category: "guard" },
  { category: "forward" },
  { category: "guard" }
];

const counts = players.reduce((acc, player) => {
  const key = player.category;
  acc[key] = (acc[key] || 0) + 1;
  return acc;
}, {});
// { guard: 2, forward: 1 }