I understand neither Randy's question not James's reply.
The direct reply would be to use...
If you just want the result Tires - 4, then...
if (field("Qty") > 0)
field("Item") + " - " + field("Qty");
JavaScript sees that the string values indicate a string type of + and concatenates them with the stringified version of the quantity.
Use...
if (field("Qty") > 0) {
field("Item") + " - " + field("Qty");
In another JavaScript field, you could have total price, if you want...
if (field"(Qty") > 0)
field("Qty") * field("Price");