Tag Archives: cucumber

Using defineParameterType() with multiple regexes in cucumber-js

In a previous post I showed how to automatically replace parameter values before they get passed to your step definition code.

Now lets say you want that replacing/parsing/transformation for single- and double-quoted string like this in your feature file

When I print "Hello world!"
When I print 'its a beautiful day!'

Well…you’re in luck! The defineParameterType() method allows you to pass an array of regexes. We can use that to support both single- and double quoted strings with the same transformation function.

There’s a big gotcha here though. The the docs say this about the transformation function

A function or method that transforms the match from the regexp. Must have arity 1 if the regexp doesn’t have any capture groups. Otherwise the arity must match the number of capture groups in regexp.

In other words, when you use an array of regexes or if your regex has multiple capture groups, the function must have the same number of parameters as the total number of capture groups of all regexes in the array.

When cucumber calls the function, each parameter contains result from the corresponding capture group in the regex/array. If a regex does not match then cucumber passes an undefined value to the corresponding parameter number of the transform function So you’ll have to check each element if its undefined/null or not before using it.

defineParameterType({
    regexp: [/'([^']*)'/, /"([^"]*)"/],
    transformer: function (singleQ, doubleQ) {
        return singleQ ? replacePlaceholders(singleQ) : replacePlaceholders(doubleQ)
    },
    name: "a_string_with_replaced_placeholders"
});

Automatic type conversion of parameters in SpecFlow

In a previous post I showed that SpecFlow can change values of parameters. This mechanism is not just limited to transforming content of string values. It can also convert the literal string in your feature file to some complex object for your step-definition code.

Let say in your feature file you want to write steps like this:

Scenario: MyScenario
    When I print this list 'A,B,C,D'

Then you might be tempted to convert the string 'A,B,C,D' to a list like this

[Binding]
public class MyBindings
{
    [StepDefinition(@"I print this list '([^']*)'") ]
    public void PrintList(string input)
    {
        var items = input.Split(',')
        foreach(var item in items)
        {
            Console.WriteLine(item)
        }
    }
}

Don’t do this…it causes the same problems as mentioned in the previous post.

Instead SpecFlow’s Step Argument Conversion lets us simplify our step definition code to this:

[Binding]
public class MyBindings
{
    [StepDefinition(@"I print this list '([^']*)'") ]
    public void PrintList(IEnumerable<string> items) 
    {
        foreach(var item in items)
        {
            Console.WriteLine(item)
        }
    }
   
    [StepArgumentTransformation]
    public IEnumerable<string> TransformStringToList(string input)
    {
        return input.Split(',');
    }
}

Automatic parsing of parameters in SpecFlow

Most implementations of cucumber can automatically replace parameter values with some run time value. Let say in your feature file you want to write steps like this:

Scenario: MyScenario
    When I print 'Hello from test case {test}' 
    When I save the value '{test}' to my database

Then you might be tempted to write these bindings (dont do this…keep reading!)

[Binding]
public class MyBindings
{
    [StepDefinition(@"I print '([^']*)'") ]
    public void PrintMessage(string Message)
    {
        string Name = ....
        Message = Message.Replace("{test}",Name)
        Console.WriteLine(Message);
    }

    [StepDefinition(@"I save '([^']*)' to my database") ]
    public void SaveToDatabase(string Message)
    {
        string Name = ....
        Message = Message.Replace("{test}",Name)
        Console.WriteLine(Message);
    }
}

This way of writing step definitions becomes a real problem as the size of your automation increases:

  1. we have to repeat the same logic in every step definition.
  2. Its easy to forget to repeat that code.
  3. New placeholders need you to find and update all places where this kind of parsing is done.
  4. Its code that’s needed but not relevant for the objective of the step definitions.

Luckily SpecFlow’s Step Argument Conversion helps us! We can simplify our binding code to this:

[Binding]
public class MyBindings
{
    [StepDefinition(@"I print '([^']*)'") ]
    public void PrintMessage(string Message) => Console.WriteLine(Message);

    [StepDefinition(@"I save '([^']*)' to my database") ]
    public void SaveToDatabase(string Message) => Console.WriteLine(Message);
    
    [StepArgumentTransformation]
    public string TransformParsedString(string input)
    {
        string Name = ...
        return input.Replace("{test}",Name);
    }
}

Other implementations of cucumber also implement this principle. Its frequently referred to using these buzzwords:

  1. Transforms / Transformations
  2. Parsing / Replacing
  3. StepArgumentTransformation

Automatically replacing/transforming input parameters in cucumber-js

Most implementations of cucumber provide a mechanism for changing literal text in the feature file to values or objects your step definition code can use. This is known as step definition or step argument transforms. Here’s how this works in cucumber-js.

Assume we have this scenario:

Scenario: Test
    When I print 'Welcome {myname}'
    And I print 'Today is {todays_date}'

And we have this step-definition.

defineStep("I print {mystring}", async function (this: OurWorld, x: string) {
    console.log(x)
});

Notice the use of {mystring} in the Cucumber expression

We can use defineParameterType() to automatically replace all placeholders.

defineParameterType({
    regexp: /'([^']*)'/,
    transformer: function (s) {
        return s
            .replace('{todays_date}', new Date().toDateString())
            .replace('{myname}', 'Gerben')
    },
    name: "mystring",
    useForSnippets: false
});

You can even use this to for objects like so:

defineParameterType({
    name: 'color',
    regexp: /red|blue|yellow/,
    transformer: s => new Color(s)
})

defineStep("I fill the canvas with the color {color}", async function (this: OurWorld, x: Color) {
    // x is an object of type Color
});

When I fill the canvas with the color red