Reduce is a function to transform an array to another something

1 Basic Example

input: votes
output: { angular: 3, react: 4, ember: 1, vanilla: 1 }

let votes = [
    "angular",
    "angular",
    "react",
    "react",
    "react",
    "angular",
    "ember",
    "react",
    "vanilla"
];

let initialVaue = {};

let reducer = function(tally, vote) {
    if(!tally[vote]) {
	tally[vote] = 1;
    } else {
	tally[vote] = tally[vote] + 1;
    }
    return tally;
};

let result = votes.reduce(reducer, initialValue);

console.log(result);

2 map and filter is also a reduce

// contents of doubled === doubleMapped
var data = [1, 2, 3];
var doubled = data.reduce(function(acc, value) {
    acc.push(value * 2);
    return acc;
}, []);
var doubleMapped = data.map(function(item) {
    return item * 2;
});

// contentes of evens === evenFiltered
var data2 = [1, 2, 3, 4, 5];
var evens = data2.reduce(function(acc, value) {
    if(value % 2 ===0) {
	acc.push(value);
    }
    return acc;
}, []);
var evenFiltered = data2.filter(function(item) {
    return (item % 2 ===0);
});
Last Updated 2017-03-10 五 10:26.
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