Question

if (typeof operand1 != "undefined" && operand1 == operand2) {
}

Above is an if statement that checks for the equality between operand1 and operand2, where operand1 may be undefined under some scenario. I wonder if the typeof operand1 != "undefined" is unnecessary.

Answer 1

Because you use the == operator and not the === operator, it’s unnecessary only if operand2 will never be null.

But you can change the if statement to use the === operator which then will pass only if operand2 “really equals” to operand1, meaning operand2 is undefined as well.

You can read more about it here:
Which equals operator (== vs ===) should be used in JavaScript comparisons?

A fiddle that shows what was written here.

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In the Data Baseworld, null(or undefined) is nothing, which means you can’t compare it to anything.

SQL example:

SELECT *
FROM   table_name t
Where  t.operand1 = null

is an error because nothing equals to null, and null doesn’t equal to anything, it just doesn’t exist.

In javascript it is not like that, you can compare things to null and undefined, but you have to be careful with TypeErrors, like with this:

var x;
x.foo; // TypeError!