How to check for undefined

[mxdvl]

There are currently three ways of checking whether a value is undefined in JavaScript:

// strict equality
if(variable === void 0) // shortest, but we seldom use it
if(variable === undefined) // we use this a lot

// type check
if(typeof variable === "undefined") // safest
 

We use the last two in our codebases, but thankfully since ES5 the global undefined cannot be redefined. It can still be shadowed, but the recommended ESLint rules prevent this via no-shadow-restricted-names.

Due to this, I believe it is not necessary to enforce this rule globally.

Appendix

To get TypeScript to play nice, you need to use type predicates and pass it as the filter. And here’s a working example.

[sndrs]

great digging, thanks!

[sndrs]

for future travellers guardian/libs#211