Bugfix-773 show better error message on JSON decoding errors

instead of dumping entire JSON, offset, line number and column numbers are displayed

js/modules/k6/http/response_test.go

@@ -178,7 +178,8 @@ func TestResponse(t *testing.T) {
 
 		t.Run("Invalid", func(t *testing.T) {
 			_, err := common.RunString(rt, sr(`http.request("GET", "HTTPBIN_URL/html").json();`))
-			assert.EqualError(t, err, "GoError: invalid character '<' looking for beginning of value")
+			//nolint:lll
+			assert.EqualError(t, err, "GoError: cannot parse json due to an error at line 1, character 2 , error: invalid character '<' looking for beginning of value")
 		})
 	})
 	t.Run("JsonSelector", func(t *testing.T) {

lib/netext/httpext/response.go

@@ -24,9 +24,11 @@ import (
 	"context"
 	"crypto/tls"
 	"encoding/json"
+	"fmt"
 	"net/http"
 	"net/http/httputil"
 
+	"github.com/loadimpact/k6/js/common"
 	"github.com/loadimpact/k6/lib"
 	"github.com/loadimpact/k6/lib/netext"
 	"github.com/pkg/errors"
@@ -59,6 +61,17 @@ const (
 	ResponseTypeNone
 )
 
+type jsonError struct {
+	line      int
+	character int
+	err       error
+}
+
+func (j jsonError) Error() string {
+	errMessage := "cannot parse json due to an error at line"
+	return fmt.Sprintf("%s %d, character %d , error: %v", errMessage, j.line, j.character, j.err)
+}
+
 // ResponseTimings is a struct to put all timings for a given HTTP response/request
 type ResponseTimings struct {
 	Duration       float64 `json:"duration"`
@@ -141,7 +154,6 @@ func (res *Response) JSON(selector ...string) (interface{}, error) {
 		}
 
 		if hasSelector {
-
 			if !res.validatedJSON {
 				if !gjson.ValidBytes(body) {
 					return nil, nil
@@ -158,11 +170,38 @@ func (res *Response) JSON(selector ...string) (interface{}, error) {
 		}
 
 		if err := json.Unmarshal(body, &v); err != nil {
-			return nil, err
+			switch t := err.(type) {
+			case *json.SyntaxError:
+				err = checkErrorInJSON(body, int(t.Offset), err)
+			default:
+				break
+			}
+			common.Throw(common.GetRuntime(res.GetCtx()), err)
 		}
 		res.validatedJSON = true
 		res.cachedJSON = v
 	}
 	return res.cachedJSON, nil
+}
+
+func checkErrorInJSON(input []byte, offset int, err error) error {
+	lf := '\n'
+	str := string(input)
+
+	// Humans tend to count from 1.
+	line := 1
+	character := 0
+
+	for i, b := range str {
+		if b == lf {
+			line++
+			character = 0
+		}
+		character++
+		if i == offset {
+			break
+		}
+	}
 
+	return jsonError{line: line, character: character, err: err}
 }