Checking whether variable is undefined about mapsapi-codestyle HOT 23 CLOSED

Agreed. Actually, I was a little surprised that the code style forbids checking for undefined via typeof.

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Global undefined is immutable:

> undefined = 1;
1
> undefined
undefined

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Which browser did you use?

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Which browser did you use?

chrome, ff, opera 12.16

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined

In modern browsers (JavaScript 1.8.5 / Firefox 4+), undefined is a non-configurable, non-writable property per the ECMAScript 5 specification.

It should work in IE9+.

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Aaaaaand what to do if you need to support IE8?

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If you write some js library and you don't known target enviroment (es5 or es3), you can

1. use typeof everywhere
2. wrap all your code in

(function (undefined) {

// code here

}());

3. use es5 to es3 transformer like es3fy

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1. is obviously better, isn't it?

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I think 3) is better. IE8 has very small percent usage. It shouldn't change style guide.
BTW In next year es6 standard should be released.

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I think 1) is better. This is more safely and readable.

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We can make clause about IE8 and typeof.

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I think it's not bad to add exception for projects which support IE8.

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@twirl, can you add this exception?

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I'd prefer to have this a common rule, not exception. It's just semantically more correct (you're checking type of variable) and safer.

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What should I do with this absolutely strange rule if I wanna do something like this:

switch(typeof myVar) {
    case 'undefined':
        doSomething1();
        break;

    case 'string':
        doSomething2();
        break;

    case 'number':
        doSomething3();
        break;
}

?

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Hm. Let's keep it at developer's discretion. IMHO this rule doesn't make a big sense.
Vote for removing the rule from the codestyle.

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@tarmolov 👍

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+1 for removing it.

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This rule does exactly the same sense as explicit type conversions via Boolean, Number, String, etc.
All types shall be cast uniformly via constructor functions and checked uniformly via typeof operator.

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This rule does exactly the same sense as explicit type conversions via Boolean, Number, String, etc.

This is not true for null. typeof null === 'object'. I don't see difference between null and undefined in this case.

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typeof null is the most famous example of JavaScript design flaw. In ES6 Type(null) == 'null'
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-terms-and-definitions-null-type

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In es6 typeof null returns 'object' http://people.mozilla.org/~jorendorff/es6-draft.html#sec-typeof-operator-runtime-semantics-evaluation

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Of course, because of backwards compatibility.

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http://es-discourse.com/t/why-typeof-is-no-longer-safe/15

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