variable assignment inside 'IF' condition

Question

in php, something like this is fine:

<?php
    if ( !$my_epic_variable = my_epic_function() ){
        // my epic function returned false
        // my epic variable is false
        die( "false" );
    }

    echo $my_epic_variable;
?>

I suppose is a shorter way of doing:

$my_epic_variable = my_epic_function();
if ( !$my_epic_variable ){
        die( "false" );
}

Can this be done is javascript? I've tried to no success, wasnt sure if there was some kind of special syntax or something

Answer 1

You can do the same in JavaScript, with one key difference. You cannot declare a (locally scoped) variable inside the if clause, you may only refer to it.

So, declare it first:

var someVar;

Then use it however you want:

if (!(someVar = someFunction())) { /* do some stuff */ }

Notice that you will also have to wrap negated expressions (!(expression)) with parentheses

This however, will not work:

if (var someVar = someFunction()) { /* NOPE */ }

Answer 2

Yes, that works fine. However, if you're inversing ( ! ), then you need to wrap the assignment in parentheses otherwise you'll get a syntax error.

function myFunc() {
    return false;
}

if(!(myVar = myFunc())) {
    console.log('true');
}    

Working example

Answer 3

It works in JS too:

var foo = null;

if ( foo = 1 ){
 // some code
}

alert (foo);  // 1

Or assignment even with a function:

var foo = null;

function getFoo(){
    return 1;
}

if ( foo = getFoo() ){
 // some code
}

alert (foo); // 1

With negation, you need to add braces:

var foo = null;

function getFoo(){
    return 1;
}

if (! (foo = getFoo()) ){
 // some code
}

alert (foo); // 1

In the last case, it is important to wrap assignment statement in parenthesis because ! is then used to check against the result of that.

Answer 4

This is the preferred method for me (in PHP), because it makes it absolutely clear that you didn't mean to use the == condition but made a mistake.

if ( ($my_epic_variable = my_epic_function()) !== NULL ){
    // my epic function returned false
    // my epic variable is false
    die( "false" );
}

In JavaScript I'd probably do:

function my_epic_function() {
    return 5;
}

var my_epic_variable;
if ( (my_epic_variable = my_epic_function()) !== null ){
    alert("we're in");
}​