How to covert file to base64

in custom javascript:

function setInputValue(element) { var file = element.files[0]; var reader = new FileReader(); reader.onloadend = function() { console.log(‘RESULT’, reader.result) console.log(this.result); buttonbase64=this.result; }
reader.readAsDataURL(file);
}

var base = buttonbase64;

in phtml file :

<input id=”upload” type=”file” onchange=”encodePdfFileAsURL(this)” name=”uploadbutton” accept=”.pdf”>