Странное поведение при настледавниии примитива в React Native?

class Alias extends String {
    toJson() {
        try {
            let value = this.concat()
            if (value) {
                value = JSON.parse(value);
            }
            return value;
        } catch (error) {
            throw new Error(`It isn't JSON string.`)
        };

    }
}
export class Aliases {
    constructor(aliases) {
        if (!aliases) throw new Error('Array of aliases are required in params.')
        aliases.forEach(alias => {
            this[alias.code] = new Alias(alias.value);
        })
    }
}