Javascript undefined, empty strings, and if statements

Question

I am not some one who has done alot in javascript. My background is in Java/C++. I am currently working on a project that is using alot of javascript however and I came across something that doesn't make sense to me but hopefully some javascript guru out there can give me a nice logical explanation.

var noDefinition = undefined;
var emptyString = "";
var noDefinitionAndEmptyString = noDefinition + emptyString;

console.log("NoDefinition");
console.log(noDefinition);

console.log("EmptyString");
console.log(emptyString);

console.log("noDefinition+emptyString");
console.log(noDefinitionAndEmptyString);

console.log("************************");
if(noDefinition == undefined)
{
    console.log("No Definition is undefined");
}


if(emptyString == undefined)
{
    console.log("emptyString is undefined");
}

if(noDefinitionAndEmptyString == undefined)
{
    console.log("noDefiniton and emptyString is undefined");
}

The code above produces the following results in my console:
[INFO] NoDefinition
[INFO]
[INFO] EmptyString
[INFO]
[INFO] noDefinition+emptyString
[INFO] undefined
[INFO] ************
[INFO] No Definition is undefined

So as you can see when I output the variables noDefinition and emptyString to the console, it produces blank output. When I concatenate them the console will produce undefined. However if I then proceed to use an if statement and compare each of them to undefined. The only if that executes is the first one.

This occurs even though when put to a console the value just shows up blank. Also the concatenation which shows up as undefined in the console fails its compare against undefined and never executes. I am confused by this and I am hoping out there some one can give me an explanation about what is going on.

possible duplicate of [Javascript: undefined !== undefined?](http://stackoverflow.com/questions/776950/javascript-undefined-undefined) — Diodeus - James MacFarlane, Mar 01 '12 at 18:59

score 2 · Answer 1 · answered Mar 01 '12 at 19:00

console.log doesn't print anything when it receives a undefined value (implementation-specific).
When you concatenate undefined with an empty string, the resulting value is "undefined". That's printed.

The previous bullet points explain your observations.

score 2 · Accepted Answer · answered Mar 01 '12 at 19:14

When you concatenate undefined with a string it becomes a string

var foo = undefined;
var bar = "";
console.log(foo+bar, typeof(foo+bar));
* output *
undefined string

ECMAScript Docs - http://www.ecmascript.org/docs.php Not sure but reading the doc on Page 141 you can find this

15.5.1 The String Constructor Called as a Function

When String is called as a function rather than as a constructor, it performs a type conversion.

Which lets me to believe why concatenation of anything with a string outputs a string

score 1 · Answer 3 · edited Mar 01 '12 at 19:45

Rewriting your example might shed more light on what actually happens when concatenating a string and an undefined value:

var noDefinition = undefined;
var emptyString = "";
var noDefinitionAndEmptyString = noDefinition + emptyString;
console.log("************************");
console.log("NoDefinition:" + " " + noDefinition + " " + typeof(noDefinition));

console.log("EmptyString:"+ " " + emptyString + " " + typeof(emptyString));

console.log("noDefinition+emptyString: " + noDefinitionAndEmptyString + " " + typeof(noDefinitionAndEmptyString));

console.log("************************");

Results in:

************************
NoDefinition: undefined undefined
EmptyString: string
noDefinition+emptyString: undefined string
************************

score 0 · Answer 4 · answered Mar 01 '12 at 19:00

0

Use this instead:

if (typeof noDefinition == "undefined") ...

answered Mar 01 '12 at 19:00

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1

THis is better approach: if (typeof noDefinition === "undefined") – Amiga500 Aug 18 '14 at 12:30

Maxim Krizhanovsky · Answer 5 · 2012-03-01T19:26:27.330

0

noDefinitionAndEmptyString is actually a string (you can use console.info(typeof noDefinitionAndEmptyString)) and its value is 'undefined'. This happens because of the concatenation - an implicit cast to string occurs, and undefined variable, casted to string, is the string 'undefined'

edited Mar 01 '12 at 19:26

answered Mar 01 '12 at 19:02

Maxim Krizhanovsky

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score 0 · Answer 6 · answered Mar 01 '12 at 19:03

0

if(noDefinition == null) covers typeOf 'undefined' and typeOf 'null', but you may want to add an and statement to make sure it's just not an empty string... like so:

if(noDefinitionAndEmptyString == null || noDefinitionAndEmptyString == "")

answered Mar 01 '12 at 19:03

Eric Hodonsky

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Javascript undefined, empty strings, and if statements

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