Get the aggregate SUM of a field using ServiceNow GlideAggregate API.

getFieldSum.js

/**
 * Get the aggregate SUM of a field using ServiceNow GlideAggregate API.
 *
 * @param {string} table Name of table to query
 * @param {string} field Name of numeric field to SUM
 * @param {string} [query] Optional encoded query string
 * @returns {number} Aggregate SUM of field
 */
function getFieldSum(table, field, query) {
    var ga = new GlideAggregate(table);
    if (query) ga.addQuery(query);
    ga.setGroup(false);
    ga.addAggregate('SUM', field);
    ga.query();
    ga.next();
    return Number(ga.getAggregate('SUM', field));
}