What is the default value of variable in for...of loop?

Question

What will be the default type of the variable in for..of loop.

for (value of [1,2,3]) // what is the default type of value
  console.log(value)

I want to know whether value type will be var/let/const.

As far I know any undeclared variable will be of type var. Is it applies to for loop variables?

Answer 1

There is no default, though I guess you could call The Horror of Implicit Globals¹ in loose mode a kind of default. :-) Don't rely on the horror of implicit globals, it's effectively a bug in the language that's fixed by strict mode. :-)

If you write the code the way you have, you have to declare the variable prior to the loop. If you don't, in loose mode, a global var is implicitly created; in strict mode (which I recommend using at all times), it's an error. If you declare the variable prior to the loop, either let or var (but not const) will work.

If you declare the variable in the loop, you can use either let or const, depending on whether you want to update the variable in the loop (and also on your preferred style):

const values = ["one", "two", "three"];

for (const value of values) {
    console.log(value);
}

for (let value of values) {
    console.log(value);
}

for (let value of values) {
    // (Note the following only changes the value of the variable, not the entry in the array)
    value = value.toUpperCase(); // You couldn't do thsi with `const`
    console.log(value);
}

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¹ (that's a post on my anemic, neglected blog)