diff --git a/langserver/completion_test.go b/langserver/completion_test.go
new file mode 100644
index 00000000..873440fa
--- /dev/null
+++ b/langserver/completion_test.go
@@ -0,0 +1,221 @@
+// Copyright 2020 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package langserver
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	v1 "github.com/prometheus/client_golang/api/prometheus/v1"
+	"github.com/prometheus/common/model"
+	promql "github.com/prometheus/prometheus/promql/parser"
+
+	"github.com/prometheus-community/promql-langserver/internal/vendored/go-tools/lsp/protocol"
+	"github.com/prometheus-community/promql-langserver/langserver/cache"
+)
+
+// This is a mock object which serves as a test helper
+type MockMetadataService struct {
+	metadata    map[string][]v1.Metadata
+	labelNames  []string
+	labelValues []model.LabelValue
+}
+
+func (m *MockMetadataService) MetricMetadata(ctx context.Context, metric string) (v1.Metadata, error) {
+	return m.metadata[metric][0], nil
+}
+
+func (m *MockMetadataService) AllMetricMetadata(ctx context.Context) (map[string][]v1.Metadata, error) {
+	return m.metadata, nil
+}
+
+func (m *MockMetadataService) LabelNames(ctx context.Context, metricName string, startTime time.Time, endTime time.Time) ([]string, error) {
+	return m.labelNames, nil
+}
+
+func (m *MockMetadataService) LabelValues(ctx context.Context, label string, startTime time.Time, endTime time.Time) ([]model.LabelValue, error) {
+	return m.labelValues, nil
+}
+
+func (m *MockMetadataService) ChangeDataSource(prometheusURL string) error {
+	return nil
+}
+
+func (m *MockMetadataService) GetURL() string {
+	return "testhost:9090"
+}
+
+func TestMetricNameCompletion(t *testing.T) {
+	s := &server{
+		metadataService: &MockMetadataService{
+			metadata: map[string][]v1.Metadata{
+				"a1": {
+					v1.Metadata{
+						Type: v1.MetricTypeCounter,
+						Help: "For completion test",
+						Unit: "bytes",
+					},
+				},
+				"b2": {
+					v1.Metadata{
+						Type: v1.MetricTypeGauge,
+						Help: "Not selected",
+						Unit: "seconds",
+					},
+				},
+				"c3": {
+					v1.Metadata{
+						Type: v1.MetricTypeUnknown,
+						Help: "Not selected",
+						Unit: "ratio",
+					},
+				},
+			},
+		},
+	}
+
+	dc := &cache.DocumentCache{}
+	dc.Init()
+	doc, err := dc.AddDocument(
+		context.Background(),
+		&protocol.TextDocumentItem{
+			URI:        "test.promql",
+			LanguageID: "promql",
+			Version:    0,
+			Text:       "",
+		})
+
+	if err != nil {
+		t.Fatalf("Error occurred when adding document: %s", err)
+	}
+
+	//TODO add compiled quries via dc.compileQuery
+
+	l := &cache.Location{
+		Doc: doc,
+		Query: &cache.CompiledQuery{
+			Pos: 0,
+		},
+		Node: &promql.Item{},
+	}
+	items := new([]protocol.CompletionItem)
+	err = s.completeMetricName(context.Background(), items, l, "a")
+
+	if err != nil {
+		t.Errorf("Error occurred when calling completeMetricName: %s.\n", err)
+	}
+
+	if len(*items) != 1 {
+		t.Errorf("Expected to have %d items, got: %d.\n", 1, len(*items))
+	}
+}
+
+func TestLabelNameCompletion(t *testing.T) {
+	s := &server{
+		metadataService: &MockMetadataService{
+			labelNames: []string{
+				"a1",
+				"a2",
+				"b3",
+				"c4",
+			},
+		},
+	}
+
+	dc := &cache.DocumentCache{}
+	dc.Init()
+	doc, err := dc.AddDocument(
+		context.Background(),
+		&protocol.TextDocumentItem{
+			URI:        "test.promql",
+			LanguageID: "promql",
+			Version:    0,
+			Text:       "",
+		})
+
+	if err != nil {
+		t.Fatalf("Error occurred when adding document: %s", err)
+	}
+
+	l := &cache.Location{
+		Doc: doc,
+		Query: &cache.CompiledQuery{
+			Pos: 0,
+		},
+		Node: &promql.Item{
+			Val: "a",
+		},
+	}
+	items := new([]protocol.CompletionItem)
+	err = s.completeLabel(context.Background(), items, l, nil)
+
+	if err != nil {
+		t.Errorf("Error occurred when calling completeLabel: %s.\n", err)
+	}
+
+	if len(*items) != 2 {
+		t.Errorf("Expected to have %d items, got: %d.\n", 2, len(*items))
+	}
+}
+
+func TestFunctionNameCompletion(t *testing.T) {
+	s := &server{
+		metadataService: &MockMetadataService{},
+	}
+
+	dc := &cache.DocumentCache{}
+	dc.Init()
+	doc, err := dc.AddDocument(
+		context.Background(),
+		&protocol.TextDocumentItem{
+			URI:        "test.promql",
+			LanguageID: "promql",
+			Version:    0,
+			Text:       "",
+		})
+
+	if err != nil {
+		t.Fatalf("Error occurred when adding document: %s", err)
+	}
+
+	l := &cache.Location{
+		Doc: doc,
+		Query: &cache.CompiledQuery{
+			Pos: 0,
+		},
+		Node: &promql.Item{},
+	}
+	items := new([]protocol.CompletionItem)
+	name := "std"
+	// expect to match 5 functions:
+	// - sort_desc
+	// - stddev_over_time
+	// - stdvar_over_time
+	// - stddev
+	// - stdvar
+	err = s.completeFunctionName(items, l, name)
+
+	if err != nil {
+		t.Errorf("Error occurred when calling completeFunctionName: %s.\n", err)
+	}
+
+	if len(*items) != 5 {
+		results := make([]string, len(*items))
+		for i, it := range *items {
+			results[i] = it.Label
+		}
+		t.Errorf("Expected to have %d items, got: %d, are: %v.\n", 5, len(*items), results)
+	}
+}