The answer
Because reduce returns a single value (not an array), so you cannot chain array methods after it. map and filter return arrays, so they can be chained. reduce is the final step that combines everything.
Chaining map, filter, and reduce in JavaScript
Why is reduce usually last in a chain in JavaScript?
More FAQs in Chaining map, filter, and reduce in JavaScript
Yes. map and filter return new arrays, so you can chain them. reduce returns a single value, so it is usually last. Example: arr.filter(n => n > 1).map(n => n * 2).reduce((a, b) => a + b, 0).
Each chain step creates a new array (multiple passes). For large arrays, a single reduce or loop may be faster. But chaining is more readable. Optimize only if profiling shows a problem.
products.filter(p => p.active).map(p => p.price).reduce((acc, price) => acc + price, 0). Filter active products, extract prices, sum them.
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