Question

111

Javascript undefined condition

rated 0 times [ 114] [ 3] / answers: 1 / hits: 31581 / 13 Years ago, mon, august 29, 2011, 12:00:00

Could somebody explain to me the difference between if(obj.x == undefined) and if(typeof obj.x == 'undefined')

In some context the first one works fine, but in other I need to use the second way.

Questions

1 - What is the difference between the two condition?

2 - Is there a best practice?

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clarissakourtneyb

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2 Years ago

answered 13 Years ago zariahdiamondz · Accepted Answer

The best practice is to not just check the truthiness but the strict equality

example

if (obj.x === undefined) {}

this use to be an issue because undefined (a global property) use to be writable, as of 1.8.5 is is non-writable, providing you with a secure comparison in ES5 spec environments.

per MDN