How to use the value-or-function.string function in value-or-function

'use strict';

var assign = require('object-assign');
var path = require('path');
var fs = require('graceful-fs');
var valueOrFunction = require('value-or-function');
var koalas = require('koalas');

var fo = require('./file-operations');

var boolean = valueOrFunction.boolean;
var number = valueOrFunction.number;
var string = valueOrFunction.string;

function prepareWrite(outFolder, file, opt, callback) {
  if (!opt) {
    opt = {};
  }

  var defaultMode = file.stat ? file.stat.mode : null;
  var options = assign({}, opt, {
    cwd: koalas(string(opt.cwd, file), process.cwd()),
    mode: koalas(number(opt.mode, file), defaultMode),
    dirMode: number(opt.dirMode, file),
    overwrite: koalas(boolean(opt.overwrite, file), true),
  });
  options.flag = (options.overwrite ? 'w' : 'wx');

  var cwd = path.resolve(options.cwd);