I think the answer is that humans are still smarter than typecheckers. I know that's not satisfying, but it's true. The typechecker typically uses just conditional statements to determine whether something can be null. But this code example feels a little contrived. I would probably restructure the code like this:
var myArray: (Array<any> | null);
if (cnd) {
elt.key = value;
myArray = [elt];
} else {
myArray = null;
}
This removes those two oddities.
Answer from jack on Stack OverflowI think the answer is that humans are still smarter than typecheckers. I know that's not satisfying, but it's true. The typechecker typically uses just conditional statements to determine whether something can be null. But this code example feels a little contrived. I would probably restructure the code like this:
var myArray: (Array<any> | null);
if (cnd) {
elt.key = value;
myArray = [elt];
} else {
myArray = null;
}
This removes those two oddities.
For question 1, you can refer to this link
For question 2, just perform a check against null like this before accessing your object
if (obj !== null) {
obj.doSomeThing();
}
or just simply like the following if your object can be null or undefined. null and undefined are what are called falsy value, so when they are used in a boolean context, they are automatically coerced to false
if (obj) {
obj.doSomeThing();
}
I’m calling a rest api using react query and it’s returning me an array of objects. I have already defined the type for that object and i’m trying to transform that response array into something else using .map or .reduce.
I have the condition for isLoading or isIdle present. So i’m assuming that after that code block, the data would be present. Is my assumption flawed?
I’m using a ternary in my return jsx function and there’s no error on my end so far. So is the question mark bad?
To clarify I am referring to the first question mark, array?.length. I saw this somewhere and can't recall what it does.
const array = [1,2,3] array?.length > 0 ? 'Array contains elements' : 'No elements within array';
Yes. As of TypeScript 3.7 (released on November 5, 2019), this feature is supported and is called Optional Chaining:
At its core, optional chaining lets us write code where TypeScript can immediately stop running some expressions if we run into a
nullorundefined. The star of the show in optional chaining is the new?.operator for optional property accesses.
Refer to the TypeScript 3.7 release notes for more details.
Prior to version 3.7, this was not supported in TypeScript, although it was requested as early as Issue #16 on the TypeScript repo (dating back to 2014).
As far as what to call this operator, there doesn't appear to be a consensus. In addition to "optional chaining" (which is also what it's called in JavaScript and Swift), there are a couple of other examples:
- CoffeeScript refers to it as the existential operator (specifically, the "accessor variant" of the existential operator):
The accessor variant of the existential operator
?.can be used to soak up null references in a chain of properties. Use it instead of the dot accessor.in cases where the base value may be null or undefined.
- C# calls this a null-conditional operator.
a null-conditional operator applies a member access,
?., or element access,?[], operation to its operand only if that operand evaluates to non-null; otherwise, it returnsnull.
- Kotlin refers to it as the safe call operator.
There are probably lots of other examples, too.
It is now possible, see answer of user "Donut".
Old answer: Standard JavaScript behaviour regarding boolean operators has something that may help. The boolean methods do not return true or false when comparing objects, but in case of OR the first value that is equal to true.
Not as nice as a single ?, but it works:
var thing = foo && foo.bar || null;
You can use as many && as you like:
var thing = foo && foo.bar && foo.bar.check && foo.bar.check.x || null;
Default values are also possible:
var name = person && person.name || "Unknown user";