Array.prototype.reduceRight()

reduceRight(callbackFn, initialValue)

The reduceRight() method applies a callback function against an accumulator and each element in the array (from right to left) to reduce it to a single value.

Syntax

reduceRight(callbackFn)
reduceRight(callbackFn, initialValue)

Parameters

Parameter	Type	Default	Description
callbackFn	Function	—	Function to execute for each element. Receives accumulator, currentValue, currentIndex, and array.
accumulator	any	—	Value accumulated across iterations. First call uses initialValue if provided, otherwise array[last].
currentValue	any	—	Value of current element.
currentIndex	number	—	Index of current element.
array	Array	—	The array being reduced.
initialValue	any	undefined	Starting value for accumulator.

Examples

Flattening nested arrays

const nested = [[1, 2], [3, 4], [5, 6]];

const flat = nested.reduceRight((acc, curr) => acc.concat(curr), [Global_Objects::eval]);

console.log(flat); // [5, 6, 3, 4, 1, 2]

Compare with reduce() which would produce [1, 2, 3, 4, 5, 6].

Difference between reduce and reduceRight

// reduce processes left to right
[1, 2, 3].reduce((acc, x) => [...acc, x * 2], [Global_Objects::eval]);
// Result: [2, 4, 6]

// reduceRight processes right to left
[1, 2, 3].reduceRight((acc, x) => [...acc, x * 2], [Global_Objects::eval]);
// Result: [6, 4, 2]

Common Patterns

Right-associative operations: When the operation is not commutative, like subtraction or division.

// Subtraction with reduce (left to right)
[10, 5, 2].reduce((a, b) => a - b); // (10 - 5) - 2 = 3

// Subtraction with reduceRight (right to left)
[10, 5, 2].reduceRight((a, b) => a - b); // (10 - (5 - 2)) = 7

When to Use reduceRight

Use reduceRight() when:

• The operation is not commutative (subtraction, division)
• You need to process from right to left
• You’re working with nested structures where order matters

See Also

• array::map — Transform each element
• array::filter — Select elements by condition