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

try

variableName in window

or

typeof window[variableName] != 'undefined'

or

window[variableName] !== undefined

or

window.hasOwnProperty(variableName)

/**
 * @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.

If you are wanting to assign a global variable only if it doesn't already exist, try:

window.someVar = window.someVar || 'hi';

or

window['someVar'] = window['someVar'] || 'hi';

/*global window */

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

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