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

问题

JSLint is not passing this as a valid code:

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

What is the correct way?

回答1:

/*global window */

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

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

回答2:

/**
 * @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.

回答3:

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

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

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

回答4:

try

variableName in window

typeof window[variableName] != 'undefined'

window[variableName] !== undefined

window.hasOwnProperty(variableName)

回答5:

I think this is actually a problem with JSLint. It will issue the following error:

Unexpected 'typeof'. Compare directly with 'undefined'.

I believe this is bad advice. In JavaScript, undefined is a global variable that is, usually, undefined. But some browsers allow scripts to modify it, like this: window.undefined = 'defined'. If this is the case, comparing directly with undefined can lead to unexpected results. Fortunately, current ECMA 5 compliant browsers do not allow assignments to undefined (and will throw an exception in strict mode).

I prefer typeof someVar === "undefined", as you posted, or someVar in window as Susei suggested.

回答6:

if (typeof someVar === "undefined") {
    var someVar = "hi!";
}

will check if someVar (local or global) is undefined.

If you want to check for a global variable you can use

if(window['someVar'] === undefined) {
    ...
}

assuming this is in a browser :)

回答7:

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".

回答8:

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){

}

来源：https://stackoverflow.com/questions/11596315/what-is-the-correct-way-to-check-if-a-global-variable-exists

标签

javascript

global-variables

jslint