Typescript with --strictNullCheck - check for invalid in a separate method
I like the compiler - strictNullCheck
I have this method:
I need to check if the headers are not null before I can use them. It's great
Now I would like to move the validation operation to a separate method like this:
But I am not getting this error:
So, there is no way to check if some object or its property is not null in a separate method?
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Use a guard type. Type protection is some expression that performs run-time checking that guarantees a type in some scope.
In your case, something like this might work (it's hard to tell since you've inserted images instead of code):
function hasHeaders(error: Response): error is Response & { headers: Headers} {
return error.headers != null
}
You can read more about types of guards in the typescript reference at https://www.typescriptlang.org/docs/handbook/advanced-types.html
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It should be pretty simple:
if(err.headers != null) {
return error.headers.get('content-type') || defaultContentType;
} else {
return defaultContentType; // or whatever
}
You can put this in your code as well hasHeaders
, however the typescript compiler may or may not throw this warning.
More details about checking for zeros:
Checking values through their "right" value
You can simply check if this value is "true", ie. this does not null
, undefined
, 0
, false
or ''
, using the value of a logical, ie, if (value) { /* do something */ }
or return value || defaultValue
or return value ? value : defaultValue
etc.
This way you can do something like this:
return error.headers ?
(error.headers.get('content-type') || defaultContentType) :
defaultContentType /* or whatever */;
It can be a little messy though if the variable names are long.
Using identifiers vs equality checks
Some people prefer to use ===
(and !==
) (identity) instead of ==
(and !=
) (equality) because it ===
is a stricter check; however null == undefined
equals true, whereas null === undefined
equals false, so use the correct one in the correct place!
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