CODEWITHSUNDEEP
javascript
Florestan Korp
Florestan Korp

Reputation: 711

Object return without assignment in JavaScript

I am currently working through the Eloquent JavaScript tutorial - Recognizing text.

I have a hard time understanding where the object assignment takes place for the else clause of the condition inside my for / of loop. The only assignment I see is if findIndex returns -1, yet when I run the code it also creates an object for all values returning true. How is this possible that it has a name value and where does this get assigned?

function countBy(array, groupName) {
    let counts = [];

    for (let item of array) {
        let name = groupName(item);
        let known = counts.findIndex(count => count.name == name);

        if (known == -1) {
            counts.push({ name, count: 1 });
        } else {               
            counts[known].count++;
        }
    }
    return counts;
}

console.log(countBy([1, 2, 3, 4, 5], n => n > 2));
// → [{name: false, count: 2}, {name: true, count: 3}]

Upvotes: 0

Views: 133

Answers (2)

Florestan Korp
Florestan Korp

Reputation: 711

findIndex() gets called on an empty array the first time ALWAYS returning -1 and therefore ALWAYS instantiating an object in the if(known == -1) clause.

Upvotes: 0

melpomene
melpomene

Reputation: 85837

The countBy function partitions an array into segments labeled with names returned from the specified callback function.

In your case the callback function is n => n > 2, which can return true or false, so your input array will be partitioned into segments named true and false.

In the code, counts is the array of segments seen so far.

let known = counts.findIndex(count => count.name == name);

checks whether we have seen a segment whose name is name before. If we have, its index is in known and we can increment its count directly. If not (known == -1), we add it as a new segment with an initial count of 1 and a name of name.

Upvotes: 2

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