How to increment an object property value if it exists, else set the initial value?

Question

How might I add check to see if a key already exists, and if does, increment the value, and if it doesn't exist, then set the initial value?

Something like this pseudo-code:

var dict = {}; var new_item = "Bill"  If new_item not in dict: dict[new_item] = 1  else:  dict[new_item] += 1 

Answer 1

dict[key] = (dict[key] || 0) + 1; 

Answer 2

@snak's way is pretty clean and the || operator makes it obvious in terms of readability, so that's good.

For completeness and trivia there's this bitwise way that's pretty slick too:

dict[key] = ~~dict[key] + 1; 

This works because ~~ will turn many non-number things into 0 after coercing to Number. I know that's not a very complete explanation, but here are the outputs you can expect for some different scenarios:

~~(null) 0 ~~(undefined) 0 ~~(7) 7 ~~({}) 0 ~~("15") 15