What's the most reliable way to check if a JavaScript variable is null?

Question

What is the mos reliable way if I want to check if the variable is null or is not present?.

There are diferent examples:

if (null == yourvar)

if (typeof yourvar != 'undefined')

if (undefined != yourvar)

Answer 1

None of the above.

You don't want to use ==, or a variety thereof, because it performs type coercion. If you really want to check whether something is explicitly null, use the === operator.

Then again, your question shows perhaps some lack of clarity of your requirements. Do you actually mean null specifically; or does undefined count too? myVar === null will certainly tell you if the variable is null, which is the question you asked, but is this really what you want?

Note that there's a lot more information in this SO question. It's not a direct duplicate, but it covers very similar principles.

Answer 2

I prefer

if (null == yourvar)

which avoids the accidental assignment in this scenario

if (yourvar = null)

Edit

JavaScript has both strict and type-converting equality comparison. For strict equality the objects being compared must have the same type and:

* Two strings are strictly equal when they have the same sequence of characters, 
  same length, and same characters in corresponding positions.
* Two numbers are strictly equal when they are numerically equal (have the 
  same number value). NaN is not equal to anything, including NaN. 
  Positive and negative zeros are equal to one another.
* Two Boolean operands are strictly equal if both are true or both are false.
* Two objects are strictly equal if they refer to the same Object.

Null and Undefined types are == (but not ===)

Read Comparison Operators

Answer 3

"undefined" is not "null". Compare

• the spoon is empty (=null)
• there is no spoon (=undefined)

some facts that can help you further

• typeof undefined is "undefined"
• typeof null is "object"
• undefined is considered equal (==) to null and vice versa
• no other value is equal (==) to either null or undefined