What is better?
if (obj === undefined) { }
vs.
if (typeof(obj) === 'undefined') { }
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Alon Gubkin
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5The question was related to Performance, and all answers are about the better one regardless of performance – Mar 11 '15 at 23:57
3 Answers
5
If you somehow can't refrain from shadowing the global undefined, or can't keep from trying to reference undeclared variables, then use:
typeof x === 'undefined'
If you adhere to good coding practices, and believe in letting broken code break, use:
x === undefined
If you want a different alternative, you can use:
x === void 0;
...where void always returns undefined, and doesn't rely on the global property.
Another safeguard you can use is to use shadowing in a good way by defining a proper undefined in a function:
(function( undefined ) {
// notice that no arguments were passed,
// so the `undefined` parameter will be `undefined`
var x;
if( x === undefined ) {
}
})();
...some people prefer to give it a different name:
(function( undef ) {
// notice that no arguments were passed,
// so the `undefined` parameter will be `undefined`
var x;
if( x === undef ) {
}
})();
user113716
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Thorough and accurate, but I'm not sure why you'd consider "letting broken code break" a better coding practice than using something else that is guaranteed to always work and is only slightly more complicated. – Tim Down Jun 26 '11 at 09:59
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1@Tim Down: Well they're personal preferences, but I don't believe in coding to accommodate poor coding practices. Broken code is an excellent instructor. Also, I find the *fix* to be visually hideous and entirely unintuitive. I understand its purpose, but then I think that once a person understands the reasons why they need it, they should no longer need it. Just my opinions. ;o) – user113716 Jun 26 '11 at 14:17
1
I would go with the second one, as "undefined" is not a reserved word. Example:
var obj = undefined;
undefined = {};
if(obj === undefined) {
console.log("undefined 1");
}
if(typeof obj === 'undefined') {
console.log("undefined 2");
}
Will only show "undefined 2" because the variable undefined can be changed.
Shaz
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This has been asked before, but the more common approach is to do this:
typeof(x) == 'undefined'
