Writing style function in OpenLayers?

ran into some problems with the = signs. this is working for me

var customStyleFunction = function(feature, resolution) {


  if(feature.get('WELL_CLASS')=== 'EXISTING') {
    strokecolor = '#020815';
  } else if(feature.get('WELL_CLASS')=== 'PROPOSED') {
    strokecolor = '#f61212';
  } else if(feature.get('WELL_CLASS')=== 'INJECTION') {
    strokecolor = '#198cff';
  }

  return [new ol.style.Style({
    image: new ol.style.Circle({

      fill: new ol.style.Fill({
        color: '#1b465a'
      }),
      stroke: new ol.style.Stroke({
        color: strokecolor,
        width: 3
      }),
      radius: 5
    })
  })];
};

and here is the layer definition:

var wellsLayer = new ol.layer.Vector({
  source: new ol.source.GeoJSON({
    projection: 'EPSG:3857',
    url: "./data/WELLS.geojson"
  }),
  name: 'Wells',
  style: customStyleFunction
});

Try this change to your layer definition:

var wellsLayer = new ol.layer.Vector({
   source: new ol.source.GeoJSON({
     projection: 'EPSG:3857',
     url: 'data/WELLS.geojson'
   }),
   style: customStyleFunction // "style:" is inserted and ',' is removed
});