Circular references will be replaced with "[Circular]".
Set to false to get double-quoted strings.
Expected to return a boolean of whether to include the property prop of the object obj in the output.
transform(obj, prop, originalResult)
Expected to return a string that transforms the string that resulted from stringifying obj[prop]. This can be used to detect special types of objects that need to be stringified in a particular way. The transform function might return an alternate string in this case, otherwise returning the originalResult.
Here's an example that uses the transform option to mask fields named "password":