"TypeError:无法调用方法'投掷'未定义的“;单元内测试

我想在单元测试中创建假错误。这是我的测试科目：

/arn.js

var fs = require('fs');
var filename = ['README.md', 'ioio.txt', 'yoyo.txt','passwd'];
 exports.readFile = function(entry,callback){
     var cntent = fs.readFile(entry, "utf8" ,function(err, data) {
              if (err) throw err;
              if (callback && typeof(callback) === "function") {
                callback(null,data.substring(0,5));
              }
     });
 }

/test/m1.js

var mock = require('mock')
var realB = require("../arn.js")
var test = require('unit.js');

var trigger1 = function(){
    throw new Error ("File not found in my computer",null);
}; 
var b = mock("../arn.js", {
    fs: {
        readFile: function (entry,encoding, callback) {
            if (entry === "cody.txt"){
                  var regEx = /txt$/;
                  if(regEx.test(entry)){
                    callback(null,'text2');
                  }
            }
            if (entry === "notfound.sure"){
               callback(trigger1);              
            }
        }  
    }
}, require);

describe('Test with static input', function(){
    it('should return text2', function(done){
        b.readFile('cody.txt', function(err,a){
            test.value(a).match('text2');
            done();
        });
    });
    it('should return Invalid file extension', function(done){
        b.readFile('notfound.sure', function(err, a){
            test.error.hasMessage('File not found in my computer');
            done();
        });
    });
    it('should return not found', function(done){
        test.asswert.throws(
            function(){
                b.readFile('notfound.sure');
            },
            function(err){
                if ( (err instanceof Error)){
                    return true;
                }
            },
            "Expected error"
        );
        done();
    });
});

输出：

Test with static input
    ✓ should return text2
    1) should return Invalid file extension
    2) should return not found

  1 passing (8ms)
  2 failing
  1) Test with static input should return Invalid file extension:
     Error: the function [Function] was thrown, throw an Error :)

  2) Test with static input should return not found:
     TypeError: Cannot call method 'throws' of undefined
      at Context.<anonymous> (test/m1.js:46:16)

我如何为他们找到解决方案？这个专业术语让我绕了好几个小时。