When declaring a variable in javascript, is the default value null?

Question

If I have a declaration as follows:

var j;

does j==null until I set it equal to something?

j==null will be true, but not because j is null. Instead it's because j is undefined, but the == operator does type coercion. The == considers null and undefined to be equal. The === operator is strict, and doesn't do any coercion. — cliffs of insanity
– cliffs of insanity, Commented May 12, 2012 at 1:38

Danilo Valente · Accepted Answer · 2014-08-13 12:18:53Z

43

No, it has the default value of undefined
But if want to use the !j condition, it will work with both the values (i.e. undefined or null)

Note that (j==null) is true, but (j===null) is false... JavaScript have "falsy" values and sometimes unexpected rules to convert values, plus fancy === operator to compare value and type at the same time.

edited Aug 13, 2014 at 12:18

answered May 12, 2012 at 1:35

Danilo Valente

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9 Comments

SoWeLie Over a year ago

Actually, j == null will evaluate to true, j === null will be false. (But yes, the default value is undefined as j === undefined will be true).

thisissami Over a year ago

so will writing k === null evaluate to true, since variable k doesn't exist? also i assume i can use the opposite, and say "if(j)", and that will evaluate to false, correct?

Dagg Nabbit Over a year ago

@thisissami no, it'll throw a ReferenceError. The tricky thing to get is that undefined is actually a valid value. There are both null and undefined, they are similar but different.

mgiuffrida Over a year ago

comment was deleted, but I'm leaving the ECMA reference on equality comparison.

Danilo Valente Over a year ago

They can be manually set or set by values returned from functions, like document.getElementById(elementThatDontExists);

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