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.

Answer 1

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