Waiting for Things

Browserless offers 4 different ways to wait for preconditions to be met on page. These are events, functions, selectors and timeouts

waitForEvent

Waits for an event to happen on the page before cotinue

Example

JSON payload

// Will fail since the event never fires
{
  "url": "https://example.com/",
  "waitForEvent": {
    "event": "fullscreenchange",
    "timeout": 5000
  }
}

cURL

curl -X POST \
  https://production-sfo.browserless.io/screenshot?token=YOUR_API_TOKEN_HERE \
  -H 'Cache-Control: no-cache' \
  -H 'Content-Type': 'application/json' \
  -d '{
  "url": "https://example.com/",
  "waitForEvent": {
    "event": "fullscreenchange",
    "timeout": 5000
  }
}' \
  --output "screenshot.png"

Javascript

import fs from 'fs/promises';

const TOKEN = "YOUR_API_TOKEN_HERE";
const url = `https://production-sfo.browserless.io/screenshot?token=${TOKEN}`;
const headers = {
  "Cache-Control": "no-cache",
  "Content-Type": "application/json"
};

const data = {
  url: "https://example.com/",
  waitForEvent: {
    event: "fullscreenchange",
    timeout: 5000
  }
};

const takeScreenshot = async () => {
  const response = await fetch(url, {
    method: 'POST',
    headers: headers,
    body: JSON.stringify(data)
  });

  const imageBuffer = await response.arrayBuffer();
  await fs.writeFile("screenshot.png", Buffer.from(imageBuffer));
  console.log("Screenshot saved as screenshot.png");
};

takeScreenshot();

Python

import requests

TOKEN = "YOUR_API_TOKEN_HERE"
url = f"https://production-sfo.browserless.io/screenshot?token={TOKEN}"
headers = {
    "Cache-Control": "no-cache",
    "Content-Type": "application/json"
}

data = {
    "url": "https://example.com/",
    "waitForEvent": {
        "event": "fullscreenchange",
        "timeout": 5000
    }
}

response = requests.post(url, headers=headers, json=data)

with open("screenshot.png", "wb") as file:
    file.write(response.content)

print("Screenshot saved as screenshot.png")

Java

import java.io.*;
import java.net.URI;
import java.net.http.*;
import java.nio.file.*;

public class TakeScreenshotWithWaitForEvent {
    public static void main(String[] args) {
        String TOKEN = "YOUR_API_TOKEN_HERE";
        String url = "https://production-sfo.browserless.io/screenshot?token=" + TOKEN;

        String jsonData = """
        {
            "url": "https://example.com/",
            "waitForEvent": {
                "event": "fullscreenchange",
                "timeout": 5000
            }
        }
        """;

        HttpClient client = HttpClient.newHttpClient();

        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create(url))
            .header("Cache-Control", "no-cache")
            .header("Content-Type", "application/json")
            .POST(HttpRequest.BodyPublishers.ofString(jsonData))
            .build();

        try {
            HttpResponse<InputStream> response = client.send(request, HttpResponse.BodyHandlers.ofInputStream());
            Files.copy(response.body(), Paths.get("screenshot.png"), StandardCopyOption.REPLACE_EXISTING);
            System.out.println("Screenshot saved as screenshot.png");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

C#

  using System;
  using System.IO;
  using System.Net.Http;
  using System.Text;
  using System.Threading.Tasks;

  class Program
  {
      static async Task Main(string[] args)
      {
          string TOKEN = "YOUR_API_TOKEN_HERE";
          string url = $"https://production-sfo.browserless.io/screenshot?token={TOKEN}";

          string jsonData = @"
            {
                ""url"": ""https://example.com/"",
                ""waitForEvent"": {
                    ""event"": ""fullscreenchange"",
                    ""timeout"": 5000
                }
            }";

          using var client = new HttpClient();

          
          var request = new HttpRequestMessage(HttpMethod.Post, url)
          {
              Content = new StringContent(jsonData, Encoding.UTF8, "application/json")
          };

          
          request.Headers.Add("Cache-Control", "no-cache");

          try
          {
              var response = await client.SendAsync(request);
              response.EnsureSuccessStatusCode();

              var imageBytes = await response.Content.ReadAsByteArrayAsync();

              await File.WriteAllBytesAsync("screenshot.png", imageBytes);

              Console.WriteLine("Screenshot saved as screenshot.png");
          }
          catch (Exception ex)
          {
              Console.WriteLine($"Error: {ex.Message}");
          }
      }
  }

waitForFunction

Waits for the provided function to return before cotinue. The function can be any valid JavaScript or EcmaScript function, and async functions are supported.

Example

JS function

async () => {
  const res = await fetch("https://jsonplaceholder.typicode.com/todos/1");
  const json = await res.json();

  document.querySelector("h1").innerText = json.title;
};

JSON payload

{
  "url": "https://example.com/",
  "waitForFunction": {
    "fn": "async()=>{let t=await fetch('https://jsonplaceholder.typicode.com/todos/1'),e=await t.json();document.querySelector('h1').innerText=e.title}",
    "timeout": 5000
  }
}

cURL

curl -X POST \
  https://production-sfo.browserless.io/screenshot?token=YOUR_API_TOKEN_HERE \
  -H 'Cache-Control: no-cache' \
  -H 'Content-Type': 'application/json' \
  -d '{
  "url": "https://example.com/",
  "waitForFunction": {
    "fn": "async()=>{let t=await fetch('https://jsonplaceholder.typicode.com/todos/1'),e=await t.json();document.querySelector('h1').innerText=e.title}",
    "timeout": 5000
  }
}' \
  --output "screenshot.png"

Javascript

import fs from 'fs/promises';

const TOKEN = "YOUR_API_TOKEN_HERE";
const url = `https://production-sfo.browserless.io/screenshot?token=${TOKEN}`;
const headers = {
  "Cache-Control": "no-cache",
  "Content-Type": "application/json"
};

const data = {
  url: "https://example.com/",
  waitForFunction: {
    fn: "async()=>{let t=await fetch('https://jsonplaceholder.typicode.com/todos/1'),e=await t.json();document.querySelector('h1').innerText=e.title}",
    timeout: 5000
  }
};

const takeScreenshot = async () => {
  const response = await fetch(url, {
    method: 'POST',
    headers: headers,
    body: JSON.stringify(data)
  });

  const imageBuffer = await response.arrayBuffer();
  await fs.writeFile("screenshot.png", Buffer.from(imageBuffer));
  console.log("Screenshot saved as screenshot.png");
};

takeScreenshot();

Python

import requests

TOKEN = "YOUR_API_TOKEN_HERE"
url = f"https://production-sfo.browserless.io/screenshot?token={TOKEN}"
headers = {
    "Cache-Control": "no-cache",
    "Content-Type": "application/json"
}

data = {
    "url": "https://example.com/",
    "waitForFunction": {
        "fn": "async()=>{let t=await fetch('https://jsonplaceholder.typicode.com/todos/1'),e=await t.json();document.querySelector('h1').innerText=e.title}",
        "timeout": 5000
    }
}

response = requests.post(url, headers=headers, json=data)

with open("screenshot.png", "wb") as file:
    file.write(response.content)

print("Screenshot saved as screenshot.png")

Java

import java.io.*;
import java.net.URI;
import java.net.http.*;
import java.nio.file.*;

public class TakeScreenshotWithWaitForFunction {
    public static void main(String[] args) {
        String TOKEN = "YOUR_API_TOKEN_HERE";
        String url = "https://production-sfo.browserless.io/screenshot?token=" + TOKEN;

        String jsonData = """
        {
            "url": "https://example.com/",
            "waitForFunction": {
                "fn": "async()=>{let t=await fetch('https://jsonplaceholder.typicode.com/todos/1'),e=await t.json();document.querySelector('h1').innerText=e.title}",
                "timeout": 5000
            }
        }
        """;

        HttpClient client = HttpClient.newHttpClient();

        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create(url))
            .header("Cache-Control", "no-cache")
            .header("Content-Type", "application/json")
            .POST(HttpRequest.BodyPublishers.ofString(jsonData))
            .build();

        try {
            HttpResponse<InputStream> response = client.send(request, HttpResponse.BodyHandlers.ofInputStream());
            Files.copy(response.body(), Paths.get("screenshot.png"), StandardCopyOption.REPLACE_EXISTING);
            System.out.println("Screenshot saved as screenshot.png");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

C#

  using System;
  using System.IO;
  using System.Net.Http;
  using System.Text;
  using System.Threading.Tasks;

  class Program
  {
      static async Task Main(string[] args)
      {
          string TOKEN = "YOUR_API_TOKEN_HERE";
          string url = $"https://production-sfo.browserless.io/screenshot?token={TOKEN}";

          string jsonData = @"
            {
                ""url"": ""https://example.com/"",
                ""waitForFunction"": {
                    ""fn"": ""async()=>{let t=await fetch('https://jsonplaceholder.typicode.com/todos/1'),e=await t.json();document.querySelector('h1').innerText=e.title}"",
                    ""timeout"": 5000
                }
            }";

          using var client = new HttpClient();

          
          var request = new HttpRequestMessage(HttpMethod.Post, url)
          {
              Content = new StringContent(jsonData, Encoding.UTF8, "application/json")
          };

          
          request.Headers.Add("Cache-Control", "no-cache");

          try
          {
              var response = await client.SendAsync(request);
              response.EnsureSuccessStatusCode();

              var imageBytes = await response.Content.ReadAsByteArrayAsync();

              await File.WriteAllBytesAsync("screenshot.png", imageBytes);

              Console.WriteLine("Screenshot saved as screenshot.png");
          }
          catch (Exception ex)
          {
              Console.WriteLine($"Error: {ex.Message}");
          }
      }
  }

waitForSelector

Wait for a selector to appear in page. If at the moment of calling the method the selector already exists, the method will return immediately. If the selector doesn't appear after the timeout milliseconds of waiting, the function will throw.

The object can have any of these values:

• selector: String, required — A valid CSS selector.
• hidden Boolean, optional — Wait for the selected element to not be found in the DOM or to be hidden, i.e. have display: none or visibility: hidden CSS properties.
• timeout: Number, optional — Maximum number of milliseconds to wait for the selector before failing.
• visible: Boolean, optional — Wait for the selected element to be present in DOM and to be visible, i.e. to not have display: none or visibility: hidden CSS properties.

Example

Rest API (Without Stealth)

JSON payload

{
  "url": "https://example.com/",
  "waitForSelector": {
    "selector": "h1",
    "timeout": 5000
  }
}

cURL

curl -X POST \
  https://production-sfo.browserless.io/screenshot?token=YOUR_API_TOKEN_HERE \
  -H 'Cache-Control: no-cache' \
  -H 'Content-Type: application/json' \
  -d '{
  "url": "https://example.com/",
  "waitForSelector": {
    "selector": "h1",
    "timeout": 5000
  }
}'

Javascript

import fs from 'fs/promises';

const TOKEN = "YOUR_API_TOKEN_HERE";
const url = `https://production-sfo.browserless.io/screenshot?token=${TOKEN}`;
const headers = {
  "Cache-Control": "no-cache",
  "Content-Type": "application/json"
};

const data = {
  url: "https://example.com/",
  waitForSelector: {
    selector: "h1",
    timeout: 5000
  }
};

const generatePDF = async () => {
  const response = await fetch(url, {
    method: 'POST',
    headers: headers,
    body: JSON.stringify(data)
  });

  const pdfBuffer = await response.arrayBuffer();
  await fs.writeFile("output.pdf", Buffer.from(pdfBuffer));
  console.log("PDF saved as output.pdf");
};

generatePDF();

Python

import requests

TOKEN = "YOUR_API_TOKEN_HERE"
url = f"https://production-sfo.browserless.io/screenshot?token={TOKEN}"
headers = {
    "Cache-Control": "no-cache",
    "Content-Type": "application/json"
}

data = {
    "url": "https://example.com/",
    "waitForSelector": {
        "selector": "h1",
        "timeout": 5000
    }
}

response = requests.post(url, headers=headers, json=data)

with open("output.pdf", "wb") as file:
    file.write(response.content)

print("PDF saved as output.pdf")

Java

import java.io.*;
import java.net.URI;
import java.net.http.*;
import java.nio.file.*;

public class GeneratePDFWithWaitForSelector {
    public static void main(String[] args) {
        String TOKEN = "YOUR_API_TOKEN_HERE";
        String url = "https://production-sfo.browserless.io/screenshot?token=" + TOKEN;

        String jsonData = """
        {
            "url": "https://example.com/",
            "waitForSelector": {
                "selector": "h1",
                "timeout": 5000
            }
        }
        """;

        HttpClient client = HttpClient.newHttpClient();

        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create(url))
            .header("Cache-Control", "no-cache")
            .header("Content-Type", "application/json")
            .POST(HttpRequest.BodyPublishers.ofString(jsonData))
            .build();

        try {
            HttpResponse<InputStream> response = client.send(request, HttpResponse.BodyHandlers.ofInputStream());
            Files.copy(response.body(), Paths.get("output.pdf"), StandardCopyOption.REPLACE_EXISTING);
            System.out.println("PDF saved as output.pdf");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

C#

using System;
using System.IO;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        string TOKEN = "YOUR_API_TOKEN_HERE";
        string url = $"https://production-sfo.browserless.io/screenshot?token={TOKEN}";

        string jsonData = @"
          {
              ""url"": ""https://example.com/"",
              ""waitForSelector"": {
                  ""selector"": ""h1"",
                  ""timeout"": 5000
              }
          }";

        using var client = new HttpClient();

        var request = new HttpRequestMessage(HttpMethod.Post, url)
        {
            Content = new StringContent(jsonData, Encoding.UTF8, "application/json")
        };

        request.Headers.Add("Cache-Control", "no-cache");

        try
        {
            var response = await client.SendAsync(request);
            response.EnsureSuccessStatusCode();
            var pdfBytes = await response.Content.ReadAsByteArrayAsync();

            await File.WriteAllBytesAsync("output.pdf", pdfBytes);
            Console.WriteLine("PDF saved as output.pdf");
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error: {ex.Message}");
        }
    }
}

BQL (Stealth)

If the /pdf API is getting blocked by bot detectors, then we would recommend trying BrowserQL.

BQL Query

mutation PDF {
  goto(url: "https://example.com/") {
    status
  }

  waitForSelector(selector: "h1" timeout: 5000) {
    selector
  }
}

cURL

curl --request POST \
  --url 'https://production-sfo.browserless.io/chromium/bql?token=YOUR_API_TOKEN_HERE' \
  --header 'Content-Type: application/json' \
  --data '{"query":"mutation PDF {\n  goto(url: \"https://example.com/\") {\n    status\n  }\n\n  waitForSelector(selector: \"h1\" timeout: 5000) {\n    selector\n  }\n}","variables":"","operationName":"PDF"}'

Javascript

const endpoint = "https://production-sfo.browserless.io/chromium/bql";
const token = "YOUR_API_TOKEN_HERE";

const options = {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({"query":"mutation PDF {\n  goto(url: \"https://example.com/\") {\n    status\n  }\n\n  waitForSelector(selector: \"h1\" timeout: 5000) {\n    selector\n  }\n}","variables":"","operationName":"PDF"})
};

const url = `${endpoint}?token=${token}`;
const response = await fetch(url, options);
const data = await response.json();

console.log(data);

Python

import requests

endpoint = "https://production-sfo.browserless.io/chromium/bql"
query_string = {
    "token": "YOUR_API_TOKEN_HERE",
}
headers = {
    "Content-Type": "application/json",
}
payload = {
    "query": "mutation PDF {\n  goto(url: \"https://example.com/\") {\n    status\n  }\n\n  waitForSelector(selector: \"h1\" timeout: 5000) {\n    selector\n  }\n}",
    "variables": None,
    "operationName": "PDF",
}

response = requests.post(endpoint, params=query_string, headers=headers, json=payload)
print(response.json())

Java

String url = "https://production-sfo.browserless.io/chromium/bql";
String token = "YOUR_API_TOKEN_HERE";
String endpoint = String.format("%s?token=%s%s%s", url, token);

HttpResponse<String> response = Unirest.post(endpoint)
    .header("Content-Type", "application/json")
    .body({"query":"mutation PDF {\n  goto(url: \"https://example.com/\") {\n    status\n  }\n\n  waitForSelector(selector: \"h1\" timeout: 5000) {\n    selector\n  }\n}","variables":"","operationName":"PDF"})
    .asString();

C#

string url = "https://production-sfo.browserless.io/chromium/bql";
string token = "YOUR_API_TOKEN_HERE";
string endpoint = $"{url}?token={token}";

var payload = new
{
    query = @"mutation PDF {
  goto(url: ""https://example.com/"") {
    status
  }

  waitForSelector(selector: ""h1"" timeout: 5000) {
    selector
  }
}",
    variables = "",
    operationName = "PDF"
};

using (var client = new HttpClient())
{
    var jsonPayload = System.Text.Json.JsonSerializer.Serialize(payload);
    var content = new StringContent(jsonPayload, Encoding.UTF8, "application/json");

    var response = await client.PostAsync(endpoint, content);
    string responseBody = await response.Content.ReadAsStringAsync();
    
    Console.WriteLine(responseBody);
}

BQL Schemas

For more details on BQL mutations, refer to the BrowserQL Schema reference pages.