How to load a function without click event in javascript

For creating a thumbnail from a video, i use this code:

<video controls>
    <source src="data/uploads/mainvideo-1601674168.mp4" type="video/mp4">                       
</video>
<canvas width="90" height="50"></canvas>                    
<button onclick="snap()">Take screenshot</button>

And the js:

<script>
// Get handles on the video and canvas elements
var video = document.querySelector('video');
var canvas = document.querySelector('canvas');
// Get a handle on the 2d context of the canvas element
var context = canvas.getContext('2d');
// Define some vars required later

var w, h, ratio;
// Define the size of the rectangle that will be filled (basically the entire element)
context.fillRect(0, 0, w, h);
// Grab the image from the video
context.drawImage(video, 0, 0, w, h);


// Add a listener to wait for the 'loadedmetadata' state so the video's dimensions can be read
video.addEventListener('loadedmetadata', function() {
    // Calculate the ratio of the video's width to height
    ratio = video.videoWidth / video.videoHeight;
    // Define the required width as 100 pixels smaller than the actual video's width
    w = 90;
    // Calculate the height based on the video's width and the ratio
    h = parseInt(w / ratio, 10);
    // Set the canvas width and height to the values just calculated
    canvas.width = w;
    canvas.height = h;          
}, false);

// Takes a snapshot of the video
function snap() {
    // Define the size of the rectangle that will be filled (basically the entire element)
    context.fillRect(0, 0, w, h);
    // Grab the image from the video
    context.drawImage(video, 0, 0, w, h);
}
</script>

Now when i click on the button, the thumbnail appears. I am not so familiar with js but how can i let appear the thumbnail immediately without clicking on the button?

I tried this at the end of the js but that doesn't work (jquery is loaded)

$(document).ready(function() {
    snap();
});