Question

In the following Java snippet, the scope of i is limited to the inside of the for loop. That’s why it causes an error. However, in the similar JS snippet, the i is apparently accessible outside of the loop. How is that possible?

Java:

for(int i=0;i<10;i++) {
    ...
}
System.out.println(i);

Output: “i is not defined”

JS:

for(var i=0;i<10;i++) { 
    ...
}
console.log(i);

Output: 10

I didn’t expect to see output from the JS. I want to know how JS differs from Java. How does JavaScript variable scope work?

Answer 1

In Javascript “local” variables have function scope, not block scope.

All local variable declarations are “hoisted” to the top of the current scope, so your code is equivalent to:

var i;
for (i = 0; i < 10; ++i) {
}
console.log(i);

Note that while the declaration is hoisted, any assignment is not. e.g. this code

function test() {
    console.log(i);  // undefined
    var i = 1;       // declaration and assignment
    console.log(i);  // 1
}

is equivalent to:

function test() {
    var i;           // declaration hoisted
    console.log(i);  // undefined
    i = 1;           // assignment still happens here
    console.log(i);  // 1
}