Usage of Javascript syntax : method(param1,param2,(something)=>{//some code},param4);

This method syntax seems to be little strange.

Here is the method declaration. Some normal javascript method implementation.

sendHttpRequest: function (aUri, aUploadData, aContentType, aExisting, aSetupChannelFunc, aFailureFunc, aUseStreamLoader = true) {
    //method implementation
}

This is the actual usage. There what does mean by using ()=>{//some statements} as a parameter?

function someFunction(){
    this.calendar.sendHttpRequest(requestUri, queryXml, MIME_TEXT_XML, null, (channel) => {
       //some statements
        return this;
    }, () => {
        if (this.setA && this.setB) {
       //some statements
        }
    }, false);

//someFunction's statements

}