Rule proposal: no-typeof-undefined

[dimaMachina]

Less code written

Fail

typeof foo === 'undefined'

Pass

foo === undefined

[fisker]

How about consistent-undefined-check, allow one of these two? Personally I always use typeof.

[fisker]

When it's undefined variable, foo === undefiend will throw

foo === undefined
VM227:1 Uncaught ReferenceError: foo is not defined
    at <anonymous>:1:1

[dimaMachina]

When it's undefined variable, foo === undefiend will throw

foo === undefined
VM227:1 Uncaught ReferenceError: foo is not defined
    at <anonymous>:1:1

in node REPL, yes, but in real projects there is always enabled no-undef ESLint rule from eslint:recommended config, so this case will be reported

[fisker]

I don't think no-undef is safe, for example, when you write code for browser, the following won't report, because AbortController is considered "defined" as globals.

if (AbortController !== undefined) {}

But when you run this code in old browsers don't support AbortController , it will throw.

if (typeof AbortController !== 'undefined') {}

This won't.

Of cause, 'AbortController' in globalThis or globalThis.AbortController are safer. But nothing wrong to use typeof.

[dimaMachina]

Ok, but we can check if the variable is declared in scope with var, let, or const keyword or variable is a function parameter.
If so, inform the suggestion, otherwise do nothing.

[sindresorhus]

This is now accepted.

[sindresorhus]

How about consistent-undefined-check, allow one of these two? Personally I always use typeof.

@fisker You are potentially silently hiding bugs by always using typeof though. I think it's generally better to only use it for things that are known to not exist, like when polyfilling. Any other times, IMHO, not using typeof is better.

[fisker]

I'm going to work on this. And I'm going to add an option to ignore global variables.

[fisker]

I've sent #1966, we'll check all typeof ANYTHING === 'undefined' by default.

When set {checkGlobalVariables: false}, the rule will ignore variables if it's not defined in file, including unresolved and known globals. typeof foo === 'undefined' and typeof Array === 'undefined' are both valid.

WDYT? @zloirock @papb

Or do you still prefer use typeof?

[zloirock]

@fisker I think that checkGlobalVariables: false should be set by default or this rule should be missed in the recommended.

[fisker]

I don't really mind, I'll use checkGlobalVariables: false anyway. @sindresorhus your call.

[sindresorhus]

I'm fine with having it false by default.

[fisker]

Changed checkGlobalVariables to false by default, also use suggestions for global variables to be safe.