reduce() is the most capable and most underused JavaScript array method. Most developers know it computes sums. Fewer realize it handles grouping, indexing, deduplication, and building any data structure from an array. This guide walks through the practical applications step by step.

Step 1: Understand the Accumulator

reduce() takes a callback and an optional initial value. The callback receives the accumulator (the running result) and the current element. Whatever the callback returns becomes the new accumulator.

array . reduce (( accumulator , current ) => { // return the new accumulator }, initialValue );

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The initial value determines what kind of result you get. Start with 0 to sum numbers. Start with {} to build an object. Start with [] to build an array.…