How do I use CoffeeScript to mock out existing code?

Question

I'd like mock out MarkdownDeep, I've the following code, in JavaScript

MarkdownDeep = new (function () {
    this.Markdown = function () {
        this.Transform = function (a) {
            return "html";
        };
    };
})();

but I'm having trouble implementing this in CoffeeScript

I tried the following

MarkdownDeep = new (->
  @Markdown = ->
    @Transform = (a) ->
      "html"
)()
window.MarkdownDeep = MarkdownDeep

but it doesn't work, specifically in my unit test markdown = new MarkdownDeep.Markdown() gives "undefined is not a function", though the JS version mocks out fine.

score 3 · Accepted Answer · edited May 23 '17 at 11:44

Your example results in the following javascript code:

var MarkdownDeep;
MarkdownDeep = new (function() {
  return this.Markdown = function() {
    return this.Transform = function(a) {
      return "html";
    };
  };
});
window.MarkdownDeep = MarkdownDeep;

The line return this.Markdown = function() { /* ... */ } makes the function the object returned by the new operator.

Writing

MarkdownDeep = new (->
  @Markdown = ->
    @Transform = (a) ->
      "html"
    return
  return
)
window.MarkdownDeep = MarkdownDeep

fixes the problem.

Addition: This answer mentions the algorithm for object construction in javascript

JS constructors can be tricky like this about their return values. — Alex Wayne, Oct 17 '11 at 19:19

Trevor Burnham · Answer 2 · 2011-10-16T23:08:09.887

CoffeeScript's implicit returns can lead to mayhem when used in conjunction with new. As others have pointed out, you could use explicit returns. Another option is to use class, which creates a function (the constructor) with no implicit return:

MarkdownDeep = new class
  constructor: ->
    @Markdown = class
      constructor: ->
        @Transform = (a) ->
          'html'

Of course, that's not very readable in this case, but as a general rule, you'll save yourself headaches by using class whenever you use new.

score 0 · Answer 3 · answered Oct 16 '11 at 22:48

You need to explicitly set a return value for your objects/classes or else it will return the member functions when creating a new instance.

JS FIDDLE

MarkdownDeep = new (->
  @Markdown = ->
    @Transform = (a) ->
      "html"
    undefined #return undefined instead of this.Transform
  undefined #return undefined instead of this.Markdown
)

markdown = new MarkdownDeep.Markdown()
alert markdown.Transform()

compiles to:

var MarkdownDeep, markdown;
MarkdownDeep = new (function() {
  this.Markdown = function() {
    this.Transform = function(a) {
      return "html";
    };
    return;
  };
  return;
});
markdown = new MarkdownDeep.Markdown();
alert(markdown.Transform());

score 0 · Answer 4 · answered Oct 16 '11 at 22:49

This is what Coffeescript gives as output

var MarkdownDeep;
MarkdownDeep = new (function() {
  return this.Markdown = function() {
    return this.Transform = function(a) {
      return "html";
    };
  };
});

The last line in every function is implicitly returned in Coffeescript. Checking this in the console yields MarkdownDeep as

function () {
    return this.Transform = function(a) {
        return "html";
    };
}

which returns a function that does not have Markdown() as a method.

score -2 · Answer 5 · answered Oct 16 '11 at 22:39

-2

CoffeeScript automatically wraps each output file in an anonymous function ((function() { ... })()). To disable this use the --bare or -b option when running coffee.

answered Oct 16 '11 at 22:39

Jordan Running

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2

This is the answer to many, many CoffeeScript questions on Stack Overflow... but not this one. :) – Trevor Burnham Oct 16 '11 at 22:48
It was worth a try. :) Thanks @Trevor. – Jordan Running Oct 16 '11 at 22:51

How do I use CoffeeScript to mock out existing code?

5 Answers5