How to check if a schema is an array in TypeScript?

function flatten<T extends Schema.Schema.Any>(x: Schema.Array$<T> | T): Schema.Array$<T> {
    return [[[HOW TO CHECK IF X IS OF SCHEMA.ARRAY$]]]] ? x : Schema.Array(x);
}

function flatten<T extends Schema.Schema.Any>(x: Schema.Array$<T> | T): Schema.Array$<T> {
    return [[[HOW TO CHECK IF X IS OF SCHEMA.ARRAY$]]]] ? x : Schema.Array(x);
}