Effect of null in JavaScript string concatenation

Question

I expected the following script to display hello, but it display nullhello:

alert(null+'hello');

Why is this so? I can change from null to "", but still would like to understand what is happening.

Bergi · Accepted Answer · 2015-02-18 13:48:41Z

10

The value null is stringified to "null", not to the empty string. That's just how it is, there is no "reason" :-)

answered Feb 18, 2015 at 13:48

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

similar to how an empty object, {}, when coerces to a string, ends up as "[object Object]" (at least in Chrome browser)

Thanks Bergi, That had me for a loop for a while. I will not forget!

Ankit Gupta · Accepted Answer · 2015-02-18 13:56:38Z

1

Javascript concatenates the two strings using the + operator. Consider the following example:

var a = null;
alert(a+'hello');

Even in the above case, the two vars are considered as strings to be concatenated.

In javascript, a default var is initialized as undefined and not null. Consider this

var b; 
alert(b+'hello');

.. This will result in undefinedhello

answered Feb 18, 2015 at 13:51

Ankit Gupta

3253 silver badges12 bronze badges