Can I declare the same variable twice in different for loops in JavaScript? [duplicate]

Answer 1

No you shouldn't. Variables declared using var have function scope, not block scope!

Redeclaring a variable using var might suggest that the variable is local to the loop/block when it isn't.

You could, however use let to declare the variable, to ensure it is block-scoped.

for (let x = 1; x <= 3; x++) {
   console.log(x)
}
    
for (let w = 65, x = String.fromCharCode(w); w <= 67; w++, x = String.fromCharCode(w)){
    console.log(x)
}

console.log(typeof x) // undefined

Answer 2

Any use of var foo in a function will scope foo to that function. It doesn't matter where in the function this takes place as var declarations are hoisted.

Additional uses of var foo in the same function are syntactically legal but will have no effect as the variable is already scoped to that function.

Since it has no effect, there is a school of thought that recommends against it (and in favour of a single var function at the very top of a function to perform all the scoping) to avoid implying that there is significance to it (to maintainers who are not entirely comfortable with this feature of JavaScript). JSLint will alert you to this usage.