What's the equivalent way of writing inline assignment and checking PHP 'logic' but in JavaScript

Question

In PHP you could write this:

if(false !== ($value = someFunctionCall())) {
 // use $value
}

How would you write an equivalent of this in JavaScript without defining

var value;

before this comparison?

Answer 1

You'd write

if (value = someFunction()) {

I don't trust my knowledge of PHP that heavily, but I suspect that explicit comparisons to false in if statements aren't necessary, as the expression is implicitly boolean. (edit — sometimes, if a function can return either an explicit boolean or some "good" value that evaluates to "falsy", then yes you do need the explicit comparison to boolean.)

edit — if you're squeamish about the ages-old confusion (or potential thereof) between = and ==, then I'd advise avoiding the construct entirely. There's nothing wrong with it, other than the fact that sometimes you want an equality comparison and sometimes you want an assignment.

edit oh also this presumes that "value" has been declared with var somewhere — if the question is about how you do the declaration in the context of an if statement, then the answer is that you cannot do that.

final edit I kind-of promise — to sum it up:

Assuming that "value" is declared:

var value;

somewhere, then:

if (false !== (value = someFunction())) { ... }

has pretty much the same semantics as the PHP original.

Answer 2

You can just have it assign when it's executed and JavaScript will automatically add the var declaration:

function foo(){
    return (Math.random() * 0.5) > 0;
}
if (false !== (f = foo())){
    alert('True!');
}
alert('f=' + f.toString());

jsFiddle Example

However, this is very bad practice. You should always declare your variables when and where you need them first. Just because PHP and JS accept this syntax doesn't mean it's good practice. The proper style would be as follows:

var f = false;
if (false !== (f = foo())){
    alert('True!');
}
alert('f=' + f.toString());

Answer 3

That would be very bad practice in JavaScript,

You would be best to do

if(!(var value = someFunctionCall())) { // use value //}

Personally i would say define it before. Your call though. If you worried about memory then define it and null it after use.