Added DOCUMENT_EXISTS function (#594)

* Added DOCUMENT_EXISTS function

* Added addition unit tests

pkg/stdlib/html/document_exists.go

@@ -0,0 +1,74 @@
+package html
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/MontFerret/ferret/pkg/runtime/core"
+	"github.com/MontFerret/ferret/pkg/runtime/values"
+	"github.com/MontFerret/ferret/pkg/runtime/values/types"
+)
+
+// DOCUMENT_EXISTS returns a boolean value indicating whether a web page exists by a given url.
+// @param {String} url - Target url.
+// @param {Object} [options] - Request options.
+// @param {Object} [options.headers] - Request headers.
+// @return {Boolean} - A boolean value indicating whether a web page exists by a given url.
+func DocumentExists(ctx context.Context, args ...core.Value) (core.Value, error) {
+	if err := core.ValidateArgs(args, 1, 2); err != nil {
+		return nil, err
+	}
+
+	if err := core.ValidateType(args[0], types.String); err != nil {
+		return nil, err
+	}
+
+	url := args[0].String()
+
+	client := http.Client{}
+	req, err := http.NewRequest("GET", url, nil)
+
+	if err != nil {
+		return values.None, err
+	}
+
+	if len(args) > 1 {
+		if err := core.ValidateType(args[1], types.Object); err != nil {
+			return nil, err
+		}
+
+		options := args[1].(*values.Object)
+
+		if options.Has("headers") {
+			headersOpt := options.MustGet("headers")
+
+			if err := core.ValidateType(headersOpt, types.Object); err != nil {
+				return nil, err
+			}
+
+			headers := headersOpt.(*values.Object)
+
+			req.Header = http.Header{}
+
+			headers.ForEach(func(value core.Value, key string) bool {
+				req.Header.Set(key, value.String())
+
+				return true
+			})
+		}
+	}
+
+	resp, err := client.Do(req.WithContext(ctx))
+
+	if err != nil {
+		return values.False, nil
+	}
+
+	var exists values.Boolean
+
+	if resp.StatusCode >= 200 && resp.StatusCode <= 299 {
+		exists = values.True
+	}
+
+	return exists, nil
+}