null checking with "and or" syntax

Question

In a jQuery special events article I found syntax that I haven't seen before:

var threshold = data && data.threshold || 1

I have seen before the following:

var threshold = data.threshold || 1

Which to the best of my knowledge means: set to data.threshold or if its value is null then set to 1. Can I please get explanation on first syntax?

Answer 1

&& has higher associativity than ||, so the first example actually means:

var threshold = (data && data.threshold) || 1;

You can read this as "set threshold to data.threshold only if data and data.threshold have truthy values, otherwise set it to 1." If data was null/undefined/etc and the code tried accessing data.threshold without first checking data, this would result in an exception. This syntax allows for checking both at once, in a compact way.