How to use choice

[koshi148]

I don't know how to use choice tag.
The code below gives me an error and I can't encode it.

const asn1 = require('asn1.js');
const Message = asn1.define('Message', function () {
    this.choice({
        aaa: this.utf8str()
    })
});
const encoded = Message.encode({
        aaa: '5'    
}, 'der');
console.log(encoded);

So I need a simple example.