previa-de-imagem-com-javascript.html

<img src="https://www.prophix.com/wp-content/uploads/2018/02/default-person.jpg" id="img" alt="" width="200" height="200">

<form method="post" enctype="multipart/form-data">
    <input type="file" id="imagem" name="imagem">
    <input type="submit" value="Enviar" name="acao">
</form>

<script>
    document.getElementById("imagem").onchange = function() {previa(event)};
    function previa(event) {
        var reader = new FileReader();
        
        reader.onload = function(){
            var img = document.getElementById('img');
            img.src = reader.result;
        };
        reader.readAsDataURL(event.target.files[0]);
    }        
</script>