Arrays

JavaScript provides three essential array methods for functional programming: map(), filter(), and reduce(). These methods process arrays without mutating the original, returning new arrays or values instead.

map()

• Creates a new array by calling a function on every element
• Always returns an array of the same length as the original
• Does not mutate the original array
• Skips empty slots in sparse arrays
• Use when you need to transform each element (e.g., doubling numbers, extracting properties)
• Common anti-pattern: calling map() without using the returned array — use forEach() for side effects instead

filter()

• Creates a new array with elements that pass a test (callback returns truthy)
• May return fewer elements than the original (or an empty array)
• Does not mutate the original array
• Use when you need to select or exclude items based on a condition

reduce()

• Executes a reducer callback on each element, accumulating into a single value
• Callback receives: (accumulator, currentValue, currentIndex, array)
• If no initial value is provided, the first element is used as the accumulator and iteration starts from index 1
• Calling reduce() on an empty array without an initial value throws a TypeError
• Use when you need to compute a single result from an array (sums, averages, grouping, flattening)

Chaining

These methods return arrays (except reduce()), so they can be chained: arr.filter(...).map(...).reduce(...). This enables a declarative, pipeline-style approach to data transformation.

Working with Objects

These are Array.prototype methods. To use them on objects, convert first with Object.keys(), Object.values(), or Object.entries().

Key Interview Distinction: map() vs forEach() map() returns a new array and is chainable. forEach() returns undefined and cannot be chained. Use map() for transformations, forEach() for side effects.