Do not assume the field will exist in all documents.

Fields may be missing from your document, or may only be supported in specific document types. Use an if test to identify problems. For example:

if (document.firstname)...

View output is case sensitive.

The value emitted by the emit() function is case sensitive. Emitting a field value of 'Martin' but specifying a key value of 'martin' will not match the data. Emitted data, and the key selection values, should be normalized to eliminate potential problems. For example:

emit(doc.firstname.toLowerCase(),null);

Number formatting

Numbers within JavaScript may inadvertently be converted and output as strings. To ensure that data is correctly formatted, the value should be explicitly converted. For example:

emit(parseInt(doc.value,10),null);

The parseInt() built-in function will convert a supplied value to an integer. The parseFloat() function can be used for floating-point numbers.