How do I use underscore to check for "undefined" in javascript?

Question

if(typeof hello! = 'undefined'){

}

This is too hard to type, would like to use underscore

Did you take two seconds to [search here](http://stackoverflow.com/search?q=%5Bjavascript%5D+underscore+check+for+undefined)? Yikes. — epascarello, Mar 05 '13 at 03:32
`hello === undefined` should work well, even without underscore .. but in many cases I find - and design code such that - a simple "false-y" conditional is correct: `if (!hello) ..` The only reason to use `typeof` here is 1) for global variables, which may not exist (ick, prefer `window.prop` to avoid ReferenceErrors!) or 2) over paranoia about `undefined` being "redefined". — , Mar 05 '13 at 03:38

score 26 · Accepted Answer · answered Mar 05 '13 at 03:32

26

According to the official documentation, you can use _.isUndefined():

_.isUndefined(window.missingVariable);
=> true

answered Mar 05 '13 at 03:32

Platinum Azure

1

It seems this only works for object properties, but not for local vars: ```_.isUndefined(hello) => Uncaught ReferenceError: hello is not defined(…)``` (Tested in Chrome 52) – nickel715 Aug 11 '16 at 08:56
3

Local variables can be checked if they're first declared: `var a; _.isUndefined(a);` Attempting to access an undeclared variable will always throw a ReferenceError unless checked via `typeof a === "undefined"`. However, this question was about how to check for undefined via Underscore.js. – Platinum Azure Aug 11 '16 at 22:32

score 1 · Answer 2 · answered Mar 05 '13 at 03:32

1

_.isUndefined(value)

answered Mar 05 '13 at 03:32

chinabuffet

2 Answers2