The Array.reduceRight() method is a built-in JavaScript function that processes each element of an array from right to left (starting from the last element to the first) and reduces it to a single output value.
Syntax
arr.reduceRight(callback(accumulator, currentValue, index, array), initialValue);Parameters
| Parameter | Description |
|---|---|
| callback | (Required) A function to execute on each element in the array. |
| accumulator | (Required in callback) The accumulated result from previous callback executions. |
| currentValue | (Required in callback) The current element being processed. |
| index | (Optional in callback) The index of the current element being processed. |
| array | (Optional in callback) The array reduceRight() is being applied on. |
| initialValue | (Optional) A value to use as the initial accumulator. If not provided, the last element of the array is used. |
Return Value
The reduceRight() method returns the final accumulated value after iterating through all elements of the array from right to left.
Example 1: Concatenating Strings in Reverse Order
let words = ['world', ' ', 'hello'];
let result = words.reduceRight((accumulator, currentValue) => accumulator + currentValue, '');
console.log(result); // Output: 'hello world'
let words = ['world', ' ', 'hello'];
let result = words.reduceRight((accumulator, currentValue) => accumulator + currentValue, '');
console.log(result); // Output: 'hello world'
let words = ['world', ' ', 'hello']; let result = words.reduceRight((accumulator, currentValue) => accumulator + currentValue, ''); console.log(result); // Output: 'hello world'
Example 2: Summing Array Elements
let arr = [1, 2, 3, 4];
let sum = arr.reduceRight((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(sum); // Output: 10
let arr = [1, 2, 3, 4];
let sum = arr.reduceRight((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(sum); // Output: 10
let arr = [1, 2, 3, 4]; let sum = arr.reduceRight((accumulator, currentValue) => accumulator + currentValue, 0); console.log(sum); // Output: 10
Example 3: Flattening a Nested Array in Reverse Order
let arr = [[5, 6], [3, 4], [1, 2]];
let flatArray = arr.reduceRight((accumulator, currentValue) => accumulator.concat(currentValue), []);
console.log(flatArray);
// Output: [5, 6, 3, 4, 1, 2]
let arr = [[5, 6], [3, 4], [1, 2]];
let flatArray = arr.reduceRight((accumulator, currentValue) => accumulator.concat(currentValue), []);
console.log(flatArray);
// Output: [5, 6, 3, 4, 1, 2]
let arr = [[5, 6], [3, 4], [1, 2]]; let flatArray = arr.reduceRight((accumulator, currentValue) => accumulator.concat(currentValue), []); console.log(flatArray); // Output: [5, 6, 3, 4, 1, 2]
Example 4: Using reduceRight() Without an Initial Value
let numbers = [10, 20, 30];
let result = numbers.reduceRight((accumulator, currentValue) => accumulator - currentValue);
console.log(result); // Output: 0
let numbers = [10, 20, 30];
let result = numbers.reduceRight((accumulator, currentValue) => accumulator - currentValue);
console.log(result); // Output: 0
let numbers = [10, 20, 30]; let result = numbers.reduceRight((accumulator, currentValue) => accumulator - currentValue); console.log(result); // Output: 0
Supported Browsers
| Browser | Support |
|---|---|
| Chrome | 3+ |
| Firefox | 3+ |
| Safari | 4+ |
| Edge | 12+ |
| Opera | 10.5+ |
| Internet Explorer | 9+ |