While reading an article on special events in jQuery, I came across a syntax that was unfamiliar to me:
var threshold = data && data.threshold || 1
I have previously encountered the following:
var threshold = data.threshold || 1
As far as I know, this means: assign the value of data.threshold or if it is null, then assign 1. Could someone please explain the first syntax to me?
&& actually has a higher precedence than ||, which means that the initial example can be interpreted as follows:
let threshold = (data && data.threshold) || 1;
In simpler terms, this can be understood as "assign the value of data.threshold to threshold only if both data and data.threshold are truthy. Otherwise, default it to 1." If data happened to be null or undefined, attempting to access data.threshold without checking for data beforehand would result in an error. This method allows for dual validation in a concise manner.
Translated into plain English
If the data has a value that is considered true
AND (using the logical AND operator)
the value of data.threshold is also considered true, then set the variable threshold to the value of data.threshold,
OR (if not),
set the variable threshold to 1.
A value that is considered true means it is not null, not undefined, and not zero (among other criteria).
&& will return the value on the left if it is false, otherwise it will return the value on the right.
Therefore, if data is false, it will return date, and then the || operator will pass in 1.
If data is not false but data.threshold is, that also evaluates as false, so the || operator still passes in 1.
Only if data.threshold is true, the || operator will pass in the left-hand side, which is data.threshold.
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