js 通过经纬度来计算两地之间的距离

// 计算距离let lat1 = 39.909let lng1 = 116.39742let lat2 = this.latitudelet lng2 = this.longitudeuni.getLocation({	type: 'gcj02',	success: res=> {	lat1 = res.latitude	lng1 = res.longitude	this.distance = this.getDistance(lat1,lng1 ,lat2 , lng2);  }})  getDistance(lat1, lng1, lat2, lng2) {  			    lat1 = lat1 || 0;  				lng1 = lng1 || 0;  				lat2 = lat2 || 0;  				lng2 = lng2 || 0;  				var rad1 = lat1 * Math.PI / 180.0;  				var rad2 = lat2 * Math.PI / 180.0;				var a = rad1 - rad2;				var b = lng1 * Math.PI / 180.0 - lng2 * Math.PI / 180.0;				var r = 6378137;				var distance = r * 2 * Math.asin(Math.sqrt(Math.pow(Math.sin(a / 2), 2) + Math.cos(rad1) * Math.cos(rad2) * Math.pow(Math.sin(b / 2), 2)));				return (distance/1000).toFixed(2);}