- Returns single value.
- Doesn't mutate original array.
- Doesn't execute the function for empty array elements.
- Syntax:
array.reduce(function(previousValue, currentValue, currentIndex, arr), initialValue);
const reduceArr = [1,2,3,4];
reduceArr.reduce((total, currentValue) => total+currentValue);
> 10
reduceArr //Doesn't mutate original array
> [1, 2, 3, 4]
reduceArr.reduce((total, currentValue) => total+currentValue, 10); // 10 is Initial value
> 20
previousValue:
value resulting from the previous call to callBackFn. On first call, initialValue
if specified otherwise array[0]
.
currentValue:
value of the current element.On the first call, the value of array[0] if initialValue
specified otherwise array[1]
.
currentIndex:
currentValue index. On first call it is 0 if initialValue
specified otherwise 1.
const array = [10, 20, 30, 40];
function reducer(previousValue, currentValue, index, array) {
const result = previousValue + currentValue;
console.log(`previous: ${previousValue}, current:${currentValue}, index: ${index}, array: ${array}`);
return result;
}
array.reduce(reducer);
> previous: 10, current: 20, index: 1, array: 10,20,30,40
> previous: 30, current: 30, index: 2, array: 10,20,30,40
> previous: 60, current: 40, index: 3, array: 10,20,30,40
> 100
Examples:
1.Flatten array using reduce.
[[1,2],[3,4],[4,5]].reduce((p,n) => p.concat(n));
> [1, 2, 3, 4, 4, 5]
2.Find instances of values in array.
['first', 'second', 'third', 'first', 'fourth', 'second'].reduce((list, name) => {
if(name in list) {
list[name]++;
} else {
list[name] = 1;
}
return list;
}, {});
> {first: 2, second: 2, third: 1, fourth: 1}
3.Group objects by property.
const obj = [
{name: 'Alex',age: 30},
{name: 'Max', age: 30},
{name: 'sony', age: 20},
{name: 'molly', age: 20}
];
function groupByProperty(array, prop) {
return array.reduce((list, current) => {
let key = current[prop];
if(!list[key]) {
list[key] = [];
}
list[key].push(current);
return list;
}, {})};
groupByProperty(obj, 'age');
> {
"20": [
{
"name": "sony",
"age": 20
},
{
"name": "molly",
"age": 20
}
],
"30": [
{
"name": "Alex",
"age": 30
},
{
"name": "Max",
"age": 30
}
]
}
4.Remove duplicate items.
['a', 'b', 'c', 'a', 'b','c'].reduce((previous, current) => {
if(previous.indexOf(current) === -1) {
previous.push(current);
}
return previous;
}, []);
> ['a', 'b', 'c']
5.Functional Composition.
const double = x => x+x;
const triple = x => 3 * x;
const pipe = (...functions) => input => functions.reduce((acc, fn, index) => {
console.log(`iteration: ${index}, accValue: ${acc}`);
return fn(acc)},input);
const multiply6 = pipe(double, triple);
multiply6(6);
> iteration: 0, accValue: 6
> iteration: 1, accValue: 12
> 36
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Top comments (4)
Cool. My intention is how
reduce
works.Hi,
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Is the reduce() function using a recursion behind the scenes ?