Explain a block of crazy JS code inside Sizzle(the CSS selector engine)
- by Andy Li
So, here is the function for pre-filtering "CHILD":
function(match){
if ( match[1] === "nth" ) {
// parse equations like 'even', 'odd', '5', '2n', '3n+2', '4n-1', '-n+6'
var test = /(-?)(\d*)n((?:\+|-)?\d*)/.exec(
match[2] === "even" && "2n" || match[2] === "odd" && "2n+1" ||
!/\D/.test( match[2] ) && "0n+" + match[2] || match[2]);
// calculate the numbers (first)n+(last) including if they are negative
match[2] = (test[1] + (test[2] || 1)) - 0;
match[3] = test[3] - 0;
}
// TODO: Move to normal caching system
match[0] = done++;
return match;
}
The code is extracted from line 442-458 of sizzle.js.
So, why is the line var test = ..., have the exec inputing a boolean? Or is that really a string?
Can someone explain it by splitting it into a few more lines of code?
