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Using JavaScript with templ

Script tags​

You can use standard script tags to load JavaScript from a URL.

templ head() {
	<head>
		<script src="https://unpkg.com/lightweight-charts/dist/lightweight-charts.standalone.production.js"></script>
	</head>
}

You can then use the imported JavaScript directly in templ.

templ body() {
	<script>
		const chart = LightweightCharts.createChart(document.body, { width: 400, height: 300 });
		const lineSeries = chart.addLineSeries();
		lineSeries.setData([
				{ time: '2019-04-11', value: 80.01 },
				{ time: '2019-04-12', value: 96.63 },
				{ time: '2019-04-13', value: 76.64 },
				{ time: '2019-04-14', value: 81.89 },
				{ time: '2019-04-15', value: 74.43 },
				{ time: '2019-04-16', value: 80.01 },
				{ time: '2019-04-17', value: 96.63 },
				{ time: '2019-04-18', value: 76.64 },
				{ time: '2019-04-19', value: 81.89 },
				{ time: '2019-04-20', value: 74.43 },
		]);
	</script>
}

Script templates​

To pass Go data to scripts, you can use a script template.

Here, the page HTML template includes a script element that loads a charting library, which is then used by the body element to render some data.

package main

script graph(data []TimeValue) {
	const chart = LightweightCharts.createChart(document.body, { width: 400, height: 300 });
	const lineSeries = chart.addLineSeries();
	lineSeries.setData(data);
}

templ page(data []TimeValue) {
	<html>
		<head>
			<script src="https://unpkg.com/lightweight-charts/dist/lightweight-charts.standalone.production.js"></script>
		</head>
		<body onload={ graph(data) }></body>
	</html>
}

The data is loaded by the backend into the template. This example uses a constant, but it could easily have collected the []TimeValue from a database.

main.go
package main

import (
	"fmt"
	"log"
	"net/http"
)

type TimeValue struct {
	Time  string  `json:"time"`
	Value float64 `json:"value"`
}

func main() {
	mux := http.NewServeMux()

	// Handle template.
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		data := []TimeValue{
			{Time: "2019-04-11", Value: 80.01},
			{Time: "2019-04-12", Value: 96.63},
			{Time: "2019-04-13", Value: 76.64},
			{Time: "2019-04-14", Value: 81.89},
			{Time: "2019-04-15", Value: 74.43},
			{Time: "2019-04-16", Value: 80.01},
			{Time: "2019-04-17", Value: 96.63},
			{Time: "2019-04-18", Value: 76.64},
			{Time: "2019-04-19", Value: 81.89},
			{Time: "2019-04-20", Value: 74.43},
		}
		page(data).Render(r.Context(), w)
	})

	// Start the server.
	fmt.Println("listening on :8080")
	if err := http.ListenAndServe(":8080", mux); err != nil {
		log.Printf("error listening: %v", err)
	}
}

script elements are templ Components, so you can also directly render the Javascript function, passing in Go data, using the @ expression:

package main

import "fmt"

script printToConsole(content string) {
	console.log(content)
}

templ page(content string) {
	<html>
		<body>
		  @printToConsole(content)
		  @printToConsole(fmt.Sprintf("Again: %s", content))
		</body>
	</html>
}

The data passed into the Javascript funtion will be JSON encoded, which then can be used inside the function.

main.go
package main

import (
	"fmt"
	"log"
	"net/http"
	"time"
)

func main() {
	mux := http.NewServeMux()

	// Handle template.
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		// Format the current time and pass it into our template
		page(time.Now().String()).Render(r.Context(), w)
	})

	// Start the server.
	fmt.Println("listening on :8080")
	if err := http.ListenAndServe(":8080", mux); err != nil {
		log.Printf("error listening: %v", err)
	}
}

After building and running the executable, running curl http://localhost:8080/ would render:

Output
<html>
	<body>
		<script type="text/javascript">function __templ_printToConsole_5a85(content){console.log(content)}</script>
		<script type="text/javascript">__templ_printToConsole_5a85("2023-11-11 01:01:40.983381358 +0000 UTC")</script>
		<script type="text/javascript">__templ_printToConsole_5a85("Again: 2023-11-11 01:01:40.983381358 +0000 UTC")</script>
	</body>
</html>