What is JavascriptExecutor in Selenium? When do you use it?

// Cast WebDriver to JavascriptExecutor
JavascriptExecutor js = (JavascriptExecutor) driver;

// Execute JS — no return value
js.executeScript("window.scrollTo(0, 500);");

// Execute JS — with return value
String title = (String) js.executeScript("return document.title;");
Long count    = (Long)   js.executeScript("return document.querySelectorAll('.item').length;");

// Pass element as argument to JS
WebElement btn = driver.findElement(By.id("submit"));
js.executeScript("arguments[0].click();", btn);
// arguments[0] = first passed argument (btn)

// Some elements reject regular .click() due to:
// - covered by overlay
// - hidden but exists in DOM
// - Angular intercepting click

WebElement btn = driver.findElement(By.id("submit"));
js.executeScript("arguments[0].click();", btn);

// Scroll to specific element
WebElement element = driver.findElement(By.id("footer-section"));
js.executeScript("arguments[0].scrollIntoView(true);", element);

// Scroll to bottom of page
js.executeScript("window.scrollTo(0, document.body.scrollHeight);");

// Scroll by pixels
js.executeScript("window.scrollBy(0, 300);");   // scroll down 300px

// Scroll element into center of viewport
js.executeScript("arguments[0].scrollIntoView({behavior:'smooth', block:'center'});", element);

// For date pickers, read-only fields, custom components
WebElement dateInput = driver.findElement(By.id("datepicker"));
js.executeScript("arguments[0].value = '2025-12-31';", dateInput);

// If you also need to trigger change event
js.executeScript(
    "arguments[0].value = arguments[1];" +
    "arguments[0].dispatchEvent(new Event('change'));",
    dateInput, "2025-12-31");

// Get computed style
String color = (String) js.executeScript(
    "return window.getComputedStyle(arguments[0]).backgroundColor;", element);

// Check if element is in viewport
Boolean isVisible = (Boolean) js.executeScript(
    "var rect = arguments[0].getBoundingClientRect();" +
    "return (rect.top >= 0 && rect.bottom <= window.innerHeight);", element);

// Get innerHTML
String html = (String) js.executeScript("return arguments[0].innerHTML;", element);

// Remove readonly attribute
js.executeScript("arguments[0].removeAttribute('readonly');", element);

// Remove disabled attribute
js.executeScript("arguments[0].removeAttribute('disabled');", element);

// Create an alert via JS (for testing alert handlers)
js.executeScript("alert('Test alert');");

// Or get localStorage values
String token = (String) js.executeScript("return localStorage.getItem('auth_token');");
js.executeScript("localStorage.setItem('auth_token', arguments[0]);", "test_token_abc");
js.executeScript("localStorage.clear();");

// Turn element border red temporarily — useful during debugging
public void highlight(WebElement el) {
    String originalStyle = el.getAttribute("style");
    js.executeScript(
        "arguments[0].style.border = '3px solid red';", el);
    Thread.sleep(500);
    js.executeScript(
        "arguments[0].setAttribute('style', arguments[1]);", el, originalStyle);
}

// Wait for async JS to complete (e.g., AJAX response)
String result = (String) js.executeAsyncScript(
    "var callback = arguments[arguments.length - 1];" +
    "fetch('/api/data').then(r => r.json()).then(d => callback(d.status));");
System.out.println("API status: " + result);