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.

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