JavaScriptでのnullっぽい値の処理

null.js

var zero = 0;
var nullString = "";

var nullValue = null;
var undefinedValue = undefined;
var nanValue = NaN;

console.log("## xxx.toString");
console.log(zero + ", " + nullString + ", " + nullValue + ", " + undefinedValue + ", " + nanValue);
console.log("## ! xxx");
console.log(!zero + ", " + !nullString + ", " + !nullValue + ", " + !undefinedValue + ", " + !nanValue);
console.log("## !! xxx");
console.log(!!zero + ", " + !!nullString + ", " + !!nullValue + ", " + !!undefinedValue + ", " + !!nanValue);

console.log("## NaN について");
console.log(typeof nanValue);
console.log(-50 < nanValue);

console.log("## xxx == null による比較");
console.log(zero == null);
console.log(nullString == null);
console.log(nullValue == null);
console.log(undefinedValue == null);
console.log(nanValue == null);

output.txt

## xxx.toString
0, , null, undefined, NaN
## ! xxx
true, true, true, true, true
## !! xxx
false, false, false, false, false
## NaN について
number
false
## xxx == null による比較
false
false
true
true
false