Sums the values of multiple fields in a GeoJSON into a new field.

sumfields.js

/* Functions for use with GeoJSONs */
/*
  Pass data like so: var newGeoJson = sumFields(inputFC, inputFields, outputField)
    Input FC can be a point, line, polygon.  Use the variable name assigned to the GeoJSON, not the L.geoJson variable
    inputFields can be a single field or an array of fields, formatted like so: [value1, value2, ..., valueN]
    outputField is the name of the output field
*/
  
/* Sum Fields
These two functions are used to sum multiple columns of data from one GeoJSON together.
They are based on the Sum function of TurfJS but modified to handle data from only one dataset instead of 2.
*/
//sumFields - receives an input feature class, an array of input fields that will be summed, and the name of a field that will be added to the GeoJSON
function sumFields(polyFC, inFields, outField) {
	var values = [];
	polyFC.features.forEach(function(poly) {
	  values.length = 0;
		for (var i = 0; i < inFields.length; i++) {
			values.push(poly.properties[inFields[i]]);
		}
		poly.properties[outField] = sum(values);
	});
	return polyFC;
}
//sum - receives an array of values from sumFields and adds them together, returning the numeric value back to sumFields.
function sum(x) {
	var value = 0;
	for (var i = 0; i < x.length; i++) {
		value += x[i];
	}
	return value;
}