Question

Can someone explain why the alert returns “undefined” instead of “hello”?

window.onload = function() {  
    var a = 'hello';  
    alert(window.a);  
}

Answer 1

“Named variables are defined with the var statement. When used inside of a function, var defines variables with function-scope.” – (source)

To be accessible globally, and particularly to make a a member of the window object, alter your code in this way:

var a; // defined in the global scope
window.onload = function() {  
    a = 'hello'; // initialized
    alert(window.a);  
}

Or in this way:

var b = 'world'; //defined and initialized in the global scope
window.onload = function() {  
    alert(window.b);  
}