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<button type="submit" class="btn btn-sm btn-primary" disabled>Upload file</button>

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$('input[type=file]').change(function(){
    if($('input[type=file]').val()==''){
        $('button').attr('disabled',true)
    } 
    else{
      $('button').attr('disabled',false);
    }
})

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<input placeholder="Enter some text" name="name" id='input'/>
<button id='button' disabled>Réserver</button>

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let inputElt = document.getElementById('input');
let btn = document.getElementById('button');

inputElt.addEventListener("input", function(){
  btn.disabled = (this.value === '');
})