gistfile1.js

#!/usr/bin/env node

function makeAverages (data) {

    // JS has no block scope only function scope, so it is better to
    // declare all variables at the top of the function.
    var measurements = [],
        categories = {},
        out_rows = [],
        measurement, i, row, category_name, measurement_name, is_error,
        category, average;

    // Loop through all lines in the table and add values and count errors, etc.
    for (i = 0; i < data.length; i++) {

        // Just for easier reading
        row = data[i];
        category_name = row.category;
        measurement_name = row.measurement;
        is_error = row.value == "ERROR";
        category = categories[category_name];

        // Add measurement if not yet added
        if (measurements.indexOf(measurement_name) == -1) {
            measurements.push(measurement_name);
        }

        // Add category if not yet added
        if (!category) {
            category = {};
            categories[category_name] = category;
        }

        // Add a new measurement with default data if not yet added
        measurement = category[measurement_name];
        if (!measurement) {
            measurement = { amount: 0, errors: 0, value: 0 };
            category[measurement_name] = measurement;
        }

        // Populate measurement adding one when counting
        // or the value if it is not an error
        measurement.amount += is_error ? 0 : 1;
        measurement.errors += is_error ? 1 : 0;
        measurement.value += is_error ? 0 : row.value;
    }

    // Add title row
    row = ["Category"];

    // Loop through measurements and add them to the title row
    for (i = 0; i < measurements.length; i++) {
        measurement_name = measurements[i];
        row.push(measurement_name + " average");
        row.push(measurement_name + " errors");
    }
    out_rows.push(row);

    // Add a row per category
    for (category_name in categories) {

        // Make new row and add the category name
        row = [category_name];

        // Loop through all measurements, even if the category doesn't
        // have such a measurement
        for (i = 0; i < measurements.length; i++) {
            measurement_name = measurements[i];
            measurement = categories[category_name][measurement_name];

            // calculate average
            average = 0;
            if (measurement && measurement.amount > 0) {
                average = measurement.value / measurement.amount;
            }

            // Add average and error count to this row
            if (measurement) {

                row.push(average);
                row.push(measurement.errors);

            } else {
                // If this category has no such measurement then add
                // just hyphens as average and error count

                row.push("-");
                row.push("-");
            }
        }

        out_rows.push(row);
    }

    return out_rows;
}

var data = [
    { category: "A", measurement: "a", value: 2 },
    { category: "A", measurement: "b", value: "ERROR" },
    { category: "A", measurement: "a", value: "ERROR" },
    { category: "B", measurement: "a", value: "ERROR" },
    { category: "B", measurement: "b", value: 3 },
    { category: "B", measurement: "b", value: 5 },
    { category: "B", measurement: "c", value: 7.12 },
    { category: "B", measurement: "c", value: 77 },
    { category: "B", measurement: "c", value: "ERROR" },
];

console.log(makeAverages(data));