Summarizing with reduce in JavaScript

Another common thing to do with arrays is to compute a single value from them. Our recurring example, summing a collection of numbers, is an instance of this. Another example is finding the script with the most characters.

The higher-order operation that represents this pattern is called reduce (sometimes also called fold). It builds a value by repeatedly taking a single element from the array and combining it with the current value. When sum­ming numbers, you’d start with the number zero and, for each element, add that to the sum.

The parameters to reduce are, apart from the array, a combining func­tion and a start value. This function is a little less straightforward than filter and map, so take a close look at it:

function reduce(array, combine, start) {

let current = start;

for (let element of array) {

current = combine(current, element);


return current;


console.log(reduce([1, 2, 3, 4], (a, b) => a + b, 0));

// → 10

The standard array method reduce, which of course corresponds to this function, has an added convenience. If your array contains at least one ele­ment, you are allowed to leave off the start argument. The method will take the first element of the array as its start value and start reducing at the sec­ond element.

console.log([1, 2, 3, 4].reduce((a, b) => a + b));

// → 10

To use reduce (twice) to find the script with the most characters, we can write something like this:

function characterCount(script) {

return script.ranges.reduce((count, [from, to]) => {

return count + (to – from);

}, 0);


console.log(SCRIPTS.reduce((a, b) => {

return characterCount(a) < characterCount(b) ? b : a; }));

// → {name: “Han”, …}

The characterCount function reduces the ranges assigned to a script by summing their sizes. Note the use of destructuring in the parameter list of the reducer function. The second call to reduce then uses this to find the largest script by repeatedly comparing two scripts and returning the larger one.

The Han script has more than 89,000 characters assigned to it in the Unicode standard, making it by far the biggest writing system in the data set. Han is a script (sometimes) used for Chinese, Japanese, and Korean text. Those languages share a lot of characters, though they tend to write them differently. The (US-based) Unicode Consortium decided to treat them as a single writing system to save character codes. This is called Han unification and still makes some people very angry.

Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming,

No Starch Press; 3rd edition.

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