Guide to creating a personalized matcher in Jasmine

Trying to create a Custom Matcher that checks for a specific number of occurrences of a particular string within another string. Here's my approach:

var customMatcher = {
    checkOccurrences: function (expectedString, expectedCount) {
        return {
            compare: function(actualString){
                actualString.match(new RegExp(expectedString, "g") || []).length == expectedCount;
            }
        }
    }
}

Encountering an error when running the code:

TypeError: Cannot read property 'pass' of undefined

The test setup looks like this:

expect(inputString).checkOccurrences("&", 2);

The expectation is for the input string to have exactly two occurrences of "&". What could be the issue here?

javascripttestingjasminematcher

Answer №1

According to the documentation found on Jasmine's website:

The function used for comparison should return an object with a pass property that is a boolean indicating whether the matcher was successful (true) or unsuccessful (false).

Make sure to provide an object with these specific properties.

Give this code a try:

var customMatcher: {
    toContainTimes: function () {
        return {
            compare: function(actualValue, expectedResult){
                var expected = expectedResult[0];
                var num = expectedResult[1];
                var result = {
                    pass: true,
                    message: ''
                }
                result.pass = actualValue.match(new RegExp(expected, "g") || []).length === num;

                if(!result.pass)
                    result.message = 'Please insert error message here, which will be displayed when the Jasmine spec fails';

                return result;
            }
        }
    }
}

Usage: 

expect(queryString).toContainTimes(["&", 2]);

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