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

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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?

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?

• javascript
• regex
• actionscript-3

Share Improve this question Follow edited Mar 12, 2010 at 5:06 YOU 124k3434 gold badges190190 silver badges222222 bronze badges asked Mar 12, 2010 at 4:51 Lauri OherdLauri Oherd 1,40311 gold badge1313 silver badges1515 bronze badges 1

• 8 Pro tip: You will never find a bug in a language or a tool. Only super cool programmers do that. – Marius Commented Mar 12, 2010 at 4:57

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2 Answers 2

Sorted by: Reset to default 11

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"]

null is an object, and when testing against objects (non-string), its first converted to string, then its giving you that result.

You could try with /Number/.test(Number) or /String/.test(String), which would return true too.

Probably String(null) is being called, which is 'null'

String(Number) will give

function Number() {
    [native code]
}

and /function Number/.test(Number) return true too

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