Page 100 - JavaScript

See Arrays are Objects for a detailed analysis.

Reducing values

5.1

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

5.1

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
}];

5.1

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

6

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