I am familiar with short-circuiting from other programming languages, where the second half of a boolean expression might not be evaluated if the first half evaluates to an expression sufficient to arrive at an overall value. Meaning, if the first half of an OR operation is TRUE (or the first half of an AND operation is FALSE), the second half is not evaluated.
tokens.length && (aToken = tokens.shift());
What happens here is that the first half of the evaluates to a non-zero value, the right half of the operation is evaluated, which causes the first item to be pushed off the tokens array and shoved into the aToken variable. If the tokens.length is 0, no further evaluation is necessary.
Also, if the && expression above evaluates to TRUE, the value of the ext ire boolean expression is value of the last value in the list.