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开发者_如何学运维 exec inputing a boolean? Or is that really a string?
Can someone explain it by splitting it into a few more lines of code?
The exec
method will receive a string, because the Boolean Logical Operators can return an operand, and not necessarily a Boolean
result, for example:
The Logical AND operator (&&
), will return the value of the second operand if the first is truthy:
true && "foo"; // "foo"
And it will return the value of the first operand if it is by itself falsy:
NaN && "anything"; // NaN
0 && "anything"; // 0
The Logical OR operator (||
) will return the value of the second operand, if the first one is falsy:
false || "bar"; // "bar"
And it will return the value of the first operand if it is by itself non-falsy:
"foo" || "anything"; // "foo"
Falsy values are: null
, undefined
, NaN
, 0
, zero-length string, and of course false
.
Anything else, evaluated in boolean context is truthy (will coerce to true
).
So, let's look the expression:
var test = /(-?)(\d*)n((?:\+|-)?\d*)/.exec(
match[2] === "even" && "2n" || // return '2n' if match[2] is 'even'
match[2] === "odd" && "2n+1" || // return '2n+1' if it's 'odd'
!/\D/.test(match[2]) && "0n+" + match[2]|| // return '0n+N' if it's a digit(N)
match[2] // otherwise, return the match[2] value
);
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