To check if a JavaScript function already exists in the header of a Razor .cshtml page, you can use JavaScript to check if the function is defined before attempting to create it. JavaScript allows you to check for the existence of functions using the typeof operator.

Here’s how you can do it:

  1. In your Razor .cshtml page, use JavaScript to check if the function already exists in the header section.
    // Your JavaScript function name
    string functionName = "myFunction";

    if (typeof @functionName === "function") {
        console.log("The function already exists.");
        // Do something if the function already exists
    } else {
        console.log("The function does not exist yet.");
        // Define the function here if it doesn't exist
        function @functionName() {
            // Function code here

In this example, functionName is the name of the JavaScript function you want to check for. The typeof @functionName === "function" condition checks if the function is already defined. If it is, a message is logged to the console indicating that the function already exists. If the function is not defined, you can create it by defining it in the else block.

By using this approach, you can safely ensure that your JavaScript function is defined only once in the header of your Razor .cshtml page, preventing potential conflicts or errors.

Please note that the check is performed within the client-side JavaScript, so the function existence check happens when the page is rendered in the user’s browser. If you are generating JavaScript dynamically on the server-side and adding it to the page, this check may not be needed, as the server-side code will ensure that the function is not duplicated.