Why is the following not valid JavaScript?
if (var foo = (true || false)) {
console.log(foo);
}
4 Answers
When you declare a variable in JavaScript the assignment will return the value of the new variable, so you can do something like this:
if (foo = (true||false)) console.log('Hello!');
> Hello!
Now if you call foo it will have a value of true:
console.log(foo);
> true
You cannot use the var private word, because if is a statement, not a function. If you want to be sure about the scope of your variable, then you have to declare it first:
var foo;
if (foo = (true||false)) console.log('Hello!');
> Hello!
Comments
Take a look here:
http://www.ecma-international.org/ecma-262/5.1/#sec-12.5
then here:
http://www.ecma-international.org/ecma-262/5.1/#sec-11
And conclude that the syntax isn't valid because of:
http://www.ecma-international.org/ecma-262/5.1/#sec-12.2
in relation (or rather lack thereof) to the above.
Comments
Try this :
var foo = true || false;
if (foo) {
console.log(foo);
}
Place the declaration first and then check for the condition.
Comments
You can do like this:
var foo;//declare the variable first
if (foo = (true || false)) { //then assign the value for foo
console.log(foo);
}
You cannot create variable declaration inside if statement.
answered Sep 3, 2014 at 11:54
Bhojendra Rauniyar
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trueto the console.(true||false)evaluates to true. The assignment offooevaluates to true. Theifchecks the conditional which is the result of the assignment, whcih istrue. The body of the if "should" then run. Clearly it is not valid JS, but my question is why?if (foo = true)would return true. it is just that thevarkeyword can not be used in a comparison. it's a statement, not a function.