There are a few problems here.
1: onBlur expects a callback, and you are calling renderPasswordConfirmError and using the return value, which is null.
2: you need a place to render the error.
3: you need a flag to track "and I validating", which you would set to true on blur. You can set this to false on focus if you want, depending on your desired behavior.
handleBlur: function () {
this.setState({validating: true});
},
render: function () {
return <div>
...
<input
type="password"
placeholder="Password (confirm)"
valueLink={this.linkState('password2')}
onBlur={this.handleBlur}
/>
...
{this.renderPasswordConfirmError()}
</div>
},
renderPasswordConfirmError: function() {
if (this.state.validating && this.state.password !== this.state.password2) {
return (
<div>
<label className="error">Please enter the same password again.</label>
</div>
);
}
return null;
},