// example matcher() based on field length
// if matcher() returns true, we will process the field with replacer()
let max_field_length = 30
function matcher(f_value) { 
return f_value.length > max_field_length 
}

// example replacer() that truncates matched fields
function replacer(f_value) { 
return f_value.slice(0,max_field_length) + ' REDACT'
}

// the main function that loops through all fields in the supplied obj
function loop_fields(target_field,start_string,recurse) {
    let value_type
    for (let [key, value] of Object.entries(target_field)) {
        value_type = typeof value;
        // recurse if specified, and not a string
        if (recurse && value_type == 'object' && !Array.isArray(value) && value) {
            loop_fields(target_field[key],start_string,true)
        
        // we have a string that qualifies based on matcher()
        } else if (value_type == 'string' && key.startsWith(start_string) && matcher(value)) {
            target_field[key] = replacer(value)
        } 
    }
}

// CALL THE MAIN LOOP
// __e contains the entire event 
// or specify an object, example: __e._raw or __e.records
// the second argument will be used to match field names (via startsWith)
// the last option (t/f) determines if we drill into nested objects
loop_fields(__e,'foobar',true)