<script type="text/javascript">
function func(){
}
var foo="foo";
</script>
<script type="text/javascript">
var foo="foo";
function func(){
}
</script>
At the beginning, when func is a function and foo is "undefined", but I want to know which is the first to be defined, func or foo?
Identifiers of both, variable and function declarations (and also formal parameters for function code) are made when entering the execution context, that's actually before the code execution.
In the case of variables, as you noted it, they are initialized with the undefined value at that stage, the assignment takes places after, when control reaches the assignment expression, e.g.:
var foo = "outer";
(function () {
alert(foo); // undefined
var foo = "inner";
})();
In the above code, we can see how the foo identifier holds the undefined value, that's because when entering the function execution context, the local foo variable was initialized.
That's one of the reasons why people recommend to declare your variables at the top of the function (to resemble what actually happens)
Functions declarations are also hoisted, you can actually use them before its declaration, because as we know, it was made before the code execution, for example:
func(); // "bar"
function func () {
return "bar";
}
See also:
In
var foo = "foo";
function func() {
}
there are two things going on
foo and func are declared simoultaneously.
The initialization of func is hoisted to the top. So functions are initialized before vars.
So the order of operations is
foo and func declaredfunc initialized to a Function objectfoo initialized to "foo"This is clearest in the code
var foo = bar;
function bar() {}
where after this code runs, typeof foo === 'function'.
I believe identifiers become defined as their declarations are encountered.
alert(foo); var foo = 1. This does not fail with an exception because foo has been defined, but it does not alert 1 either because foo has not been initialized.Functions in javascript are always parsed and defined first.
<script type="text/javascript">
alert('before func: ' + func); // Print function as string
alert('before somevar: ' + somevar); // Print undefined
function func() {
}
var somevar = "hello";
alert('after func: ' + func); // Print function as string
alert('after somevar: ' + somevar); // Print hello
</script>
