mojo.internal.Union(
    {{union.name}}Spec.$, '{{union.name}}',
    {
{%- for field in union.fields %}
      '{{field.name}}': {
        'ordinal': {{field.ordinal}},
        'type': {{field.kind|spec_type}},
{%-    if field.kind.is_nullable %}
        'nullable': true,
{%-    endif %}
      },
{%- endfor %}
    });

export interface {{union.name}} {
{%- for field in union.fields %}
  {{field.name}}?: {{field.kind|ts_type_maybe_nullable}},
{%- endfor %}
}

export enum {{union.name}}FieldTags {
{%- for field in union.fields %}
  {{field.name|to_union_field_tag_name}},
{%- endfor %}
}

export function which{{union.name}}(u: {{union.name}}): {{union.name}}FieldTags {
{#- do not use falsy checks, because that would capture falsy values like 0 -#}
{#- and boolean false -#}
{% for field in union.fields %}
{%-   set field_access = 'u.' + field.name %}
  if ({{field_access}} !== null && {{field_access}} !== undefined) {
    return {{union.name}}FieldTags.{{field.name|to_union_field_tag_name}};
  }
{%- endfor %}

{%- if union.extensible %}
  return {{union.name}}FieldTags.{{union.default_field.name|to_union_field_tag_name}};
{%- else %}
  throw new Error("no union field has been set, this is an illegal state");
{%- endif %}
}