问如果值为空，如何设置输入的默认值？jQuery

EN

$('#test').on('change blur',function(){
      if($(this).val().trim().length === 0){
        $(this).val(5);
      }
    })

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input id="test" type="text" value = "5" />

$(':text').on('blur', function(e){
   this.value = this.value.trim() || this.defaultValue;
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type='text' value='5'>

$(selector).on('change blur', function() {
    // sets the value of the element based on the current-value;
    // if the value is equal to an empty string ('') once the
    // leading and trailing white-space is removed (using 
    // String.prototype.trim()) then we set the value to the
    // defaultValue (the value held by the <input> on page-load)
    // otherwise we set it to the current-value:
    this.value = this.value.trim() === '' ? this.defaultValue : this.value;
});

function defaultIfEmpty(){
    let el = this;
    el.value = el.value.trim() === '' ? el.defaultValue : el.value;
}
selector.addEventListener('change', defaultIfEmpty);