Javascript 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;
});

Output

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);

Output

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
}];

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

Output

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

Output

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

Output

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);

Output

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;
}, []);

Output