I am having fun with the compiler --strictNullCheck option
I have this method:
I need to check if the headers are not null before i can use them. That's great
Now I would like to move the checking operation to the separate method like this:
But not I'm getting this error:
So there is no way to check if some object or the some of its property is not null in the separate method?
Use a type guard. A type guard is some expression that performs a runtime check that guarantees the type in some scope.
In your case something like this may work (its hard to tell since you pasted images instead of code):
function hasHeaders(error: Response): error is Response & { headers: Headers} {
return error.headers != null
}
You can learn more about type guards in typescript handbook at https://www.typescriptlang.org/docs/handbook/advanced-types.html
It should be pretty simple:
if(err.headers != null) {
return error.headers.get('content-type') || defaultContentType;
} else {
return defaultContentType; // or whatever
}
You could also put that in your hasHeaders code, however, the typescript compiler may or may not still throw that warning.
A couple more details about checking for nulls though:
You can just check if the value is "truthy", i.e. it is not null, undefined, 0, false, or '' by using the value as a boolean, i.e. if (value) { /* do something */ } or return value || defaultValue or return value ? value : defaultValue, etc.
In this way you can do something like this:
return error.headers ?
(error.headers.get('content-type') || defaultContentType) :
defaultContentType /* or whatever */;
Though that can be a little messy if your variable names are long.
Some people prefer to use === (and !==) (identity) instead of == (and !=) (equality), because === is a stricter check; however, null == undefined is equal to true, whereas null === undefined is equal to false, so use the correct one in the correct place!
