Reducing values

The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.

Array Sum

This method can be used to condense all values of an array into a single value:

[1, 2, 3, 4].reduce(function(a, b) { return a + b;
});
// → 10

Optional second parameter can be passed to reduce(). Its value will be used as the first argument (specified as a) for the first call to the callback (specified as function(a, b)).

[2].reduce(function(a, b) { 
    console.log(a, b); // prints: 1 2 
    return a + b;
}, 1); // → 3

Flatten Array of Objects

The example below shows how to flatten an array of objects into a single object.

var array = [{ key: 'one',
value: 1 }, {
key: 'two',
value: 2 }, {
    key: 'three',
value: 3 }];

Version ≤ 5.1

array.reduce(function(obj, current) { 
    obj[current.key] = current.value; 
    return obj;
}, {});

Version ≥ 6

array.reduce((obj, current) => Object.assign(obj, {
  [current.key]: current.value
}), {});

Version ≥ 7

array.reduce((obj, current) => ({...obj, [current.key]: current.value}), {});

Note that the Rest/Spread Properties is not in the list of finished proposals of ES2016. It isn't supported by ES2016. But we can use babel plugin babel-plugin-transform-object-rest-spread to support it.

All of the above examples for Flatten Array result in:

{
    one: 1,
    two: 2,
    three: 3 
}

Version ≥ 5.1

Map Using Reduce

As another example of using the initial value parameter, consider the task of calling a function on an array of items, returning the results in a new array. Since arrays are ordinary values and list concatenation is an ordinary function, we can use reduce to accumulate a list, as the following example demonstrates:

function map(list, fn) {
return list.reduce(function(newList, item) {
return newList.concat(fn(item)); }, []);
}
// Usage:
map([1, 2, 3], function(n) { return n * n; }); // → [1, 4, 9]

Note that this is for illustration (of the initial value parameter) only, use the native map for working with list transformations (see Mapping values for the details).

Version ≥ 5.1

Find Min or Max Value

We can use the accumulator to keep track of an array element as well. Here is an example leveraging this to find the min value

var arr = [4, 2, 1, -10, 9]
arr.reduce(function(a, b) { return a < b ? a : b
}, Infinity); // → -10

Version ≥ 6

Find Unique Values

Here is an example that uses reduce to return the unique numbers to an array. An empty array is passed as the second argument and is referenced by prev.

var arr = [1, 2, 1, 5, 9, 5];
arr.reduce((prev, number) => { if(prev.indexOf(number) === -1) {
    prev.push(number);
  }
return prev; }, []);
// → [1, 2, 5, 9]