To check if a variable(a) has the value null, use the following code:
if (a === null) // Best way to do it!
if (a == null) // This does return true if a is null but it returns true also if a is undefined... so avoid it
To check if a variable(a) has the value undefined, use the following code:
if (typeof a === "undefined") // This works
if (a === undefined) // This works too
if (a == undefined) // This does return true if a is undefined but it returns true also if a is null... so avoid it
JavaScript has two sets of equality operators: === and ==. They mostly work exactly the same ie, if the operands are of the same type and have the same value, then === produces true & so does ==. But if the operands are of different types, then === produces false & == says true. The simplest reason is that == is also a type converter.
Here are some examples of === & == in action:
true == 1 // true
true === 1 // false, because they are of a different type
1 == "1" // true, automatic type conversion for value only
1 === "1" // false, because they are of a different type
null == undefined // true
null === undefined // false
== is also notoriously inconsistent. See the below examples:
'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined // true
' \t\r\n ' == 0 // true
Best practice is to avoid == and use === consistently in your code.