（Javascript）为三元运算符问题设置的多个条件

Question

for (var days = 1; days <= 31; ++days) {
    console.log(
        (days == (1, 31, 21) ? days + 'st':'') ||
        (days == (2, 22) ? days + 'nd':'') ||
        (days == (3, 23) ? days + 'rd':'') ||
        days + 'th'
    );
}

Trying to display (1st, 2nd, 3rd) (21st, 22nd, 23rd) (31st) (multiple th) however I'm getting a strange result here, I'm not quite sure what I've done wrong, any help will be appreciated.

I did try to google and figure this out, promise, just would appreciate a relatively detailed explanation as to why its behaving strange.

Answer 1

You've typed in some code that is syntactically correct, but it doesn't mean anything like what you apparently expect it to mean.

This:

(days == (1, 31, 21) ? days + 'st':'')

is in effect exactly the same as

(days == 21 ? days + 'st':'')

The (1, 31, 21) subexpression involves the comma operator, which allows a sequence of expressions (possibly with side-effects) to be evaluated. The overall value is the value of the last expression.

If you want to compare a value to a list of possibilities, in general you can

• use a sequence of == (or === ) comparisons connected by || ;
• use a switch statement with groups of case clauses;
• use .indexOf() to look for a value in an array.

In this particular case, I'd probably make an array containing the suffixes and then index into it:

var suffixes = Array.apply(null, new Array(32)).map(function() { return "th"; });
suffixes[2] = suffixes[22] = "nd";
suffixes[1] = suffixes[21] = suffixes[31] = "st";
suffixes[3] = suffixes[23] = "rd";

Then you can just index into the array by day number to get the suffix.