class {
    onCreate(input) {
        this.state = { mode: 'sign-in' };
    }
    setType(value) {
        this.state.mode = value;
    }
}

<div>
    <if(state.mode === 'sign-in')>
        <h1>Sign in</h1>
        Create a new account? <a on-click('setType', 'sign-up')>Sign up</a>
    </if>
    <else>
        <h1>Sign up</h1>
        Already have an account? <a on-click('setType', 'sign-in')>Sign in</a>
    </else>
</div>