1 files changed, 106 insertions, 2 deletions

diff --git a/js_lib.go b/js_lib.go
index abfe07c..ce4500a 100644
--- a/js_lib.go
+++ b/js_lib.go
@@ -9,13 +9,27 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"log"
+	"mime"
 	"net/http"
 	gourl "net/url"
+	"os"
+	"path/filepath"
 	"strings"
+
+	"golang.org/x/sync/errgroup"
 )
 
-func NewJSLibrary(client *http.Client) Imports {
-	return Imports{
+func NewJSLibrary(client *http.Client) (imports Imports, wait func()) {
+	downloads := &errgroup.Group{}
+
+	// Allow 5 parallel downloads. Why 5?
+	// Docker downloads 3 layers in parallel.
+	// My Chrome downloads up to 6 files in parallel.
+	// 5 feels like a reasonable number.
+	downloads.SetLimit(5)
+
+	im := Imports{
 		"flyscrape": map[string]any{
 			"parse": jsParse(),
 		},
@@ -23,8 +37,11 @@ func NewJSLibrary(client *http.Client) Imports {
 			"get":      jsHTTPGet(client),
 			"postForm": jsHTTPPostForm(client),
 			"postJSON": jsHTTPPostJSON(client),
+			"download": jsHTTPDownload(client, downloads),
 		},
 	}
+
+	return im, func() { downloads.Wait() }
 }
 
 func jsParse() func(html string) map[string]any {
@@ -85,6 +102,93 @@ func jsHTTPPostJSON(client *http.Client) func(url string, data any) map[string]a
 	}
 }
 
+func jsHTTPDownload(client *http.Client, g *errgroup.Group) func(url string, dst string) {
+	fileExists := func(name string) bool {
+		_, err := os.Stat(name)
+		return err == nil
+	}
+
+	isDir := func(path string) bool {
+		if strings.HasSuffix(path, "/") {
+			return true
+		}
+		if filepath.Ext(path) == "" {
+			return true
+		}
+		s, err := os.Stat(path)
+		return err == nil && s.IsDir()
+	}
+
+	suggestedFilename := func(url, contentDisp string) string {
+		filename := filepath.Base(url)
+
+		if contentDisp == "" {
+			return filename
+		}
+
+		_, params, err := mime.ParseMediaType(contentDisp)
+		if err != nil {
+			return filename
+		}
+
+		name, ok := params["filename"]
+		if !ok || name == "" {
+			return filename
+		}
+
+		return filepath.Base(name)
+	}
+
+	return func(url string, dst string) {
+		g.Go(func() error {
+			req, err := http.NewRequest("GET", url, nil)
+			if err != nil {
+				log.Printf("error downloading file %q: %v", url, err)
+				return nil
+			}
+			req.Header.Add(HeaderBypassCache, "true")
+
+			resp, err := client.Do(req)
+			if err != nil {
+				log.Printf("error downloading file %q: %v", url, err)
+				return nil
+			}
+			defer resp.Body.Close()
+
+			if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+				log.Printf("error downloading file %q: unexpected status code %d", url, resp.StatusCode)
+				return nil
+			}
+
+			dst, err = filepath.Abs(dst)
+			if err != nil {
+				log.Printf("error downloading file %q: abs path failed: %v", url, err)
+				return nil
+			}
+
+			if isDir(dst) {
+				name := suggestedFilename(url, resp.Header.Get("Content-Disposition"))
+				dst = filepath.Join(dst, name)
+			}
+
+			if fileExists(dst) {
+				return nil
+			}
+
+			os.MkdirAll(filepath.Dir(dst), 0o755)
+			f, err := os.Create(dst)
+			if err != nil {
+				log.Printf("error downloading file %q: file save failed: %v", url, err)
+				return nil
+			}
+			defer f.Close()
+
+			io.Copy(f, resp.Body)
+			return nil
+		})
+	}
+}
+
 func jsFetch(client *http.Client, req *http.Request) (obj map[string]any) {
 	obj = map[string]any{
 		"body":    "",