Given an array of values, finds sequences of nulls.

find-array-null-runs.js

/**
 * Given an array of values, finds sequences of nulls.
 *
 * @param data The array of values within which to search.
 *
 * @returns
 *   An array of objects with `start` and `end` properties. These hold indices
 *   of non-null values on either side of one or more null values. No object is
 *   returned for leading or trailing nulls.
 */
function findGaps(data) {
    const gaps = [];
    let lastValue = null;
    for (let i = 0; i < data.length - 1; i++) {
        if (data[i] === null || data[i+1] !== null) continue;
        for (let j = i+1; j < data.length; j++) {
            if (data[j] === null) continue;
            gaps.push({ start: i, end: j });
            i = j-1;
            break;
        }
    }
    return gaps;
}

// Sanity checks
[
    findGaps([1]), // Expected: []
    findGaps([null]), // Expected: []
    findGaps([1, 2, 3, 4, 5]), // Expected: []
    findGaps([null, 2, 3, 4, 5]), // Expected: []
    findGaps([1, 2, 3, 4, null]), // Expected: []
    findGaps([1, 2, null, 4, 5]), // Expected: [{ start: 1, end: 3}]
    findGaps([1, 2, null, null, 5]), // Expected: [{ start: 1, end: 4 }]
    findGaps([1, null, 3, null, 5]), // Expected: [{ start: 0, end: 2 }, { start: 2, end: 4 }]
    findGaps([null, 1, null, 3, null, 5, null]), // Expected: [{ start: 1, end: 3 }, { start: 3, end: 5 }]
    findGaps([null, 1, null, 3, 4, null, 5, null]), // Expected: [{ start: 1, end: 3 }, { start: 4, end: 6 }]
]