how come the output of this IIFE is 5?
(function() {
var a = b = 5;
})();
console.log(b);
I tried console.log(a) but it gives a reference error as expected how come 'b' is alive in global scope?
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Try separate assignments and variable declarations. This shouldn't work in strict mode. – zero298 Aug 27 '20 at 01:15
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I wanna understand why does it work? – Maaz Ahmad Khan Aug 27 '20 at 08:22
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i might be completely wrong, but is this related to hoisting somehow? – Maaz Ahmad Khan Aug 27 '20 at 09:32
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1It has nothing to do with hoisting – Quentin Aug 27 '20 at 09:39
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1you are not in strict mode, b is not declared in scope, so it get created on the global object(global or window). in strict mode, you will get an error when assigning to b. this is a good question and i am sure it is dublicated. – Tobias Nickel Aug 27 '20 at 09:45
4 Answers
2
This is happening simply because you're declaring b as a global variable, not a local one.
(function() {
var a = b = 5;
})();
It may look like it's being defined locally because of var, but that applies to a only.
Mitya
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Interesting question. Though it is not related to IIFE's or hoisting at all. Notice how "a" is not defined!
Your code sample
function test() {
var a = b = 5;
}
is semantically equivalent to this:
function test() {
var a = 5;
// this is essentially the same as `window.b = a`
b = a;
}
since you did not declare "a" (aka var a;) it ends up in the global scope.
In strict mode this would not work.
Patrick Hollweck
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That's because of "leaking", which means to unintentionally make locally declared variables available to the global scope. More informations about can be found here. Let's split your code:
var a = b = 5;
This means: a takes the value of b, which is 5. The variable b is implicitly declared and initialised in this case (with b = 5), and since you're not specifying its block scope (that's because var is referring to a, not b) it's bounded to the global scope.
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From documentation.
The scope of a variable declared with var is its current execution context and closures thereof, which is either the enclosing function and functions declared within it
matek997
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