Question

I recently found a piece of javascript which was evaluating null / undefined like so:

// The single & is on purpose!
if(x !== null & x !== undefined) {
    // Do this...
}

This looks like a very bad practice to me, but it seems to work.

As far am I am aware, these operators are supposed to perform two different tasks:

var x = 3; x &= 6; // No syntax errors.

var x = 3; x &&= 6; // Syntax error.

if(x == null && x == undefined) // No syntax errors.

if(x == null & x == undefined) // No syntax errors, but wrong operator usage?

Can anyone shed some light on this please?

Answer 1

When used like that, both the operators & and && work as logical operators (even if the & operator isn’t actually a logical operator). The practical difference is that && uses short circuit evaluation, and & doesn’t.

There is no problem using the & operator, as long as it’s possible to evaluate the second operand even if the first operand evaluates to false.

The && operator can be used to keep the second operand from being evaluated, when the first operand determines if the second operand is possible to evaluate. For example:

if (x != null && x.prop == 42) { ... }

This is shorter than having to check one, then the other:

if (x != null) {
  if (x.prop == 42) { ... }
}

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There is no &&= operator, as using short circuit evaluation in that case doesn’t make sense.