What is the scope of a 'var' declared inside a function?

Question

Could someone clarify for me please... I've read that creating a variable using reserved word 'var' makes that variable public but how can that be if the variable was created inside a function:

$('#timeIn').timepicker({ 'scrollDefaultNow': true });
    $('#timeIn').on('change', function() {
        var numIn = $('#timeIn').timepicker(('getSecondsFromMidnight'));
        var inHours = {
            hours: (numIn/86400)*24,
            getter: function() {return this.hours;}
        };
        timeIn = $('#timeIn').val();
        inArray.push(timeIn);
        events.push(timeIn);
});

In this example the variables numIn & inHours are only known within that onChange method, correct? If that is the case what would the global declaration look like? The 'timeIn' is globally scoped but without manipulation I only get a string representation back. What are my options for getting a computable time back as return.

See [JavaScript Variable Scope](http://stackoverflow.com/questions/500431/javascript-variable-scope) — , Sep 06 '12 at 17:33
@pst you are correct sir but part of I wanted most was a confirmed rebuttal of the reserve word 'var', when and where to use it and what can see it-but there is a ton of information on scope at your link — MCGRAW, Sep 06 '12 at 17:38

score 10 · Answer 1 · answered Sep 06 '12 at 17:14

10

Using the word var within the function binds it to that function's scope.

Not using the word var makes it public in all functions and all scopes

answered Sep 06 '12 at 17:14

AlanFoster

8,156
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3

Clarification: not using `var`, binds the name to the _global object_, which in browsers is the `window` object, but in other environments (e.g. Node) might be different. – lanzz Sep 06 '12 at 17:16
Also, an interesting minor detail: `var` in "global scope" has the side-effect of making it non-`delete`'able .. (it is resolvable as a property, but treated as a variable) – Sep 06 '12 at 17:21
@pst you can't say `delete window.someVar;`? – jbabey Sep 06 '12 at 17:22
@jbabey `var x = 2; delete x; window.x` this differs from `y = 2; delete y; window.y` (assuming global scope in both examples) .. it's just a little detail, but `var` in global scope does do something. (My preference/style dictates explicitly using `window.globalProp` and not using `var` in a global context, but that's another story ..) – Sep 06 '12 at 17:22

score 5 · Accepted Answer · answered Sep 06 '12 at 17:16

JavaScript uses function scope - every variable can be seen only from within the same function or a scope higher than it.

An implicit global variable is what happens when you use a variable without first declaring it. In compiled languages this would result in a compilation error, but javascript silently declares the variable as a property of the global object (in a browser this is the window object)

$('#timeIn').on('change', function() {
    var numIn; // only available from inside this anonymous handler function
    ... snip ...
    timeIn = $('#timeIn').val(); // this is an implicit global since it has not been declared anywhere
    // an explicit global, for example's sake
    window.someVar = 'foo';
});

With javascript v1.7 you can also establish block scopes via the let keyword:

let(a = 5, b = 1) {
    // a and b are scoped to this block
    console.log(a+b); // 6
}
console.log(a+b); // error

score 2 · Answer 3 · answered Sep 06 '12 at 17:15

2

There are only 3 scopes in JavaScript.

x = 1; <- x is in global scope

var, when used outside of a function, will also create a global:

<script type="text/javascript">var x = 1;</script> <- x is in global scope

function () { var x = 1; } <- x is in function scope

for (let i = 0; i < 5; i += 1) {} <- x is in block scope

answered Sep 06 '12 at 17:15

Lucas Green

3,951
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A function body is a block, so it is still block scope. – lanzz Sep 06 '12 at 17:17
@pst, that's the whole context. – Lucas Green Sep 06 '12 at 17:18
2

@lanzz No. Because C has "block scope"; e.g. `if (y) {int x; x = 1;}` – Sep 06 '12 at 17:18
1

@Mythril My point is that there is not enough context given in this reply :) It is not the assignment that makes it global, it is the *lack* of being scoped otherwise. Including more context in this reply will remove the ambiguity. – Sep 06 '12 at 17:18

What is the scope of a 'var' declared inside a function?

3 Answers3