I'm surprised that the following code works:
while(fred !== "stop"){
var fred = prompt("Should I stop or go?")
};
I'd understand this in a do while loop:
do {
code to be executed at least once
}
while (condition);
How is JavaScript able to set up the condition before the function has declared fred to be a variable?
Other questions I've read pertain to var declarations inside the conditional.
From Mozilla documentation:
Variable declarations, wherever they occur, are processed before any code is executed.
and
Because variable declarations (and declarations in general) are processed before any code is executed, declaring a variable anywhere in the code is equivalent to declaring it at the top. This also means that a variable can appear to be used before it's declared. This behavior is called "hoisting", as it appears that the variable declaration is moved to the top of the function or global code.
and a warning:
It's important to point out that the hoisting will affect the variable declaration, but not its value's initialization. The value will be indeed assigned when the assignment statement is reached:
JavaScript is a bit odd when declaring variables with var; they are declared in the function scope, not the block scope, like other languages. If there is a var inside your function, anywhere, in the end the declaration is pulled up to the beginning (not the instantiation); so your code becomes:
function () {
var fred;
while(fred !== "stop"){
fred = prompt("Should I stop or go?")
};
}
When just declared, the variable exists and receives the value of undefined. If you want the more sensible block scoped variables, use let/const. Then you'd get the behavior you expected.
Before declaring fred, fred is undefined, so undefined !== "stop", which is true.
undefined" != undeclared
console.log(fred)beforepromptstatement, you will notice thatnullis printed bazillion times before yourpromptis even processed. That saidfredis already hoisted since it has function scope.var xis in the middle of the function, and that (and hoisting) is central to the question (and the answers answer this question).