What is the correct way to check if a global variable exists?

Question

JSLint is not passing this as a valid code:

/* global someVar: false */
if (typeof someVar === \"undefined\") {
    var someVar = \"hi!\";
}

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Answer 1

/*global window */

if (window.someVar === undefined) {
    window.someVar = 123456;
}

if (!window.hasOwnProperty('someVar')) {
    window.someVar = 123456;
}

Answer 2

bfavaretto is incorrect.

Setting the global undefined to a value will not alter tests of objects against undefined. Try this in your favorite browsers JavaScript console:

var udef; var idef = 42;
alert(udef === undefined); // Alerts "true".
alert(idef === undefined); // Alerts "false".
window.undefined = 'defined';
alert(udef === undefined); // Alerts "true".
alert(idef === undefined); // Alerts "false".

This is simply due to JavaScript ignoring all and any values attempted to be set on the undefined variable.

window.undefined = 'defined';
alert(window.undefined); // Alerts "undefined".

Answer 3

/**
 * @param {string} nameOfVariable
 */
function globalExists(nameOfVariable) {
    return nameOfVariable in window
}

It doesn't matter whether you created a global variable with var foo or window.foo — variables created with var in global context are written into window.

Answer 4

This would be a simple way to perform the check .

But this check would fail if variableName is declared and is assigned with the boolean value: false

if(window.variableName){

}