Javascript - validation, numbers only

Question

I\'m trying to get my login form to only validate if only numbers were inputted. I can it to work if the input is only digits, but when i type any characters after a number,

Answer 1

function ValidateNumberOnly()
{
if ((event.keyCode < 48 || event.keyCode > 57)) 
{
   event.returnValue = false;
}
}

this function will allow only numbers in the textfield.

Answer 2

If you are using React, just do:

<input
  value={this.state.input}
  placeholder="Enter a number"
  onChange={e => this.setState({ input: e.target.value.replace(/[^0-9]/g, '') })}
/>

<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.4.2/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.4.2/react-dom.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.21.1/babel.min.js"></script>
<script type="text/babel">
class Demo extends React.Component {
    state = {
      input: '',
    }
    
    onChange = e => {
      let input = e.target.value.replace(/[^0-9]/g, '');
      this.setState({ input });
    }
    
    render() {
        return (
          <div>
            <input
              value={this.state.input}
              placeholder="Enter a number"
              onChange={this.onChange}
            />
            <br />
            <h1>{this.state.input}</h1>
           </div>
        );
    }
}

ReactDOM.render(<Demo />, document.getElementById('root'));
</script>