lib/utils.js
function addToFields(fieldName, options) {
	if (inFields(fieldName, options)) return;
	options.fields = options.fields.concat([fieldName]);
}
function addToFields(fieldName, options) {
	if (inFields(fieldName, options)) return;
	options.fields = options.fields.concat([fieldName]);
}
function addToFields ( fieldName , options ) { if ( inFields ( fieldName , options ) ) return ; options . fields = options . fields . concat ( [ fieldName ] ) ; }
Add a field to `options.fields`. NB Clones `options.fields` before adding to it, to avoid options being mutated externally.
Add a field to options . fields . NB Clones options . fields before adding to it to avoid options being mutated externally .