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JavaScript - count occurrences of array elements

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Created by:

Rubi-Reyna

677

In this article, we would like to show you how to count occurrences of array elements using JavaScript.

Practical examples

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Example 1

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In this example, we create a reusable function that counts occurrences and stores them in the object.

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const countElements = (array) => {
    const counts = {};
    for (const element of array) {
        const count = counts[element] ?? 0;  // or:  const count = counts[element] || 0;
        counts[element] = count + 1;
    }
    return counts;
};
​
​
// Usage example:
​
const input = ['a', 'a', 'a', 'b', 'b', 'c'];
const counts = countElements(input);
​
for (const key in counts) {
    console.log(key + ' ' + counts[key]);
}

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Example 2

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In this example, we create a reusable function that counts occurrences and stores them in the object. Keys in the object are acheved using keyGetter function which lets to use a composite keys (composed of multiple properties).

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const countElements = (array, keyGetter) => {
    const counts = {};
    for (const element of array) {
        const key = keyGetter(element);
        const count = counts[key] ?? 0;  // or:  const count = counts[key] ?? 0;
        counts[key] = count + 1;
    }
    return counts;
};
​
​
// Usage example:
​
const input = [
    { x: 1, y: 1 },
    { x: 1, y: 1 },
    { x: 1, y: 2 },
    { x: 2, y: 2 },
    { x: 2, y: 2 }
];
​
const counts = countElements(input, (element) => '(' + element.x + ',' + element.y + ')');
​
for (const key in counts) {
    console.log(key + ': ' + counts[key]);
}