JSFiddle - React, Tailwind, and code Playground

HTML

<input id="s20" type="text" name="text20" value="0" maxlength="5" />

JavaScript

if(!String.prototype.trim){  
  String.prototype.trim = function(){  
    return this.replace(/^\s+|\s+$/g,'');  
  };  
}

document.getElementById('s20').onblur = function(){
    var val = this.value;
    
    if(!val.trim()) {
        this.value = '0'
    }
};