interface IDirectoryLink
{
    link_title: string
    link_desc: string
}

class DirectoryModel implements IDirectoryLink
{

    public link_title: string
    public link_desc: string

    constructor(fields: Object)
    {
        // Loops to populate class properties with fields values
        _.forOwn(fields, (value, key) => // Needs fat arrow to bind 'this'
        {
            if (value) { this[key] = value }
        })
    }
}

let directoryModel: IDirectoryLink = new DirectoryModel(fields)

let val = validation.modelValidator(directoryModel)

export let modelValidator = function (directoryModel: IDirectoryLink) // cannot find name IDirectoryLink
{
//code
}