在函数内部定义一个未定义的全局变量

Question

This is messy stuff (not my code but I'm stuck to it). A function depends on a globally defined variable.

function variableIssues(){
    alert(someGlobalString);  // alerts "foo"
}

Sometimes this globally defined variable is, undefined . In this case we want to cast it for further processing. The function is modified.

function variableIssues(){
    alert(someGlobalString); // undefined

    if (!someGlobalString){
        var someGlobalString = "bar";
    }  
}

However, if this function is now called with a defined someGlobalString, because of javascript evaluation the variable is set to undefined and always get set to bar .

function variableIssues(){
    alert(someGlobalString); // "should be foo, but javascript evaluates a 
                             // variable declaration it becomes undefined"

    if (!someGlobalString){
        var someGlobalString = "bar";
    }  
}

I would like to get some suggestions on how to handle undefined global variable. Any ideas?

Answer 1

Global variables are properties of the window object, so you can access them explicitly with window :

if (!window.someGlobalString) {
// depending on possible values, you might want:
// if (typeof window.someGlobalString === 'undefined')
    window.someGlobalString = "bar";
}

If you are using global variables, then this is better style, since it is clear what you are doing and assigning to undefined global variables wouldn't throw an error in strict mode.