javascript - Is it a bug in Ecmascript - /S/.test(null) returns true?

Question

Both in Actionscript3 and Javascript these statements give the same result:

/S/.test(null) => true  
/null/.test(null) => true  
/m/.test(null) => false  
/n/.test(null) => true  

Seems that null value is converted into string "null" in this case.

Is this a known bug in Ecmascript or am I missing something?

Answer 1

It's not a bug, but you are right, null coerces to 'null' and that behavior is defined on the spec:

1. RegExp.prototype.test(string), internally is equivalent to the expression: RegExp.prototype.exec(string) != null
2. The exec method type converts the first argument to string, using the ToString internal operation (look the Step 1 of the exec method).
3. The ToString internal operation, explicitly returns "null" when the input is of type Null.

In conclusion, in your examples, the RegExp matchs against the string 'null', so the first non-space character, in this case the letter 'n'.

var a = null+''; // 'null'
/S/.test(a); // true
(null+'').match(/S/) // ["n"]