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.

Answer 1

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