Given the scenario presented
interface fooInterface {
bar: any;
}
function(value: fooInterface | string) {
value.bar
}
An issue arises with the message: Property 'bar' does not exist on type '(fooInterface | string)'
I seem to be making a mistake. Essentially, what I aim to convey is that the 'value' can be either an object adhering to fooInterface or a string.
What approach should I take to achieve this?
Your assistance is appreciated
Avoid using value.bar because it may not be completely safe. Although it could potentially be safe (if value is a string), the compiler cannot guarantee this and will restrict you from using .bar unless it is certain. To solve this issue, consider implementing a type guard:
if (typeof value !== "string") {
value.bar
// The compiler allows this within the if statement since value is now of type 'fooInterface',
// as TypeScript understands that it's not a string but must be a fooInterface.
}
Experiment with this concept in the typescript playground: observe that only one instance of value.bar fails, demonstrating pinpointed errors.
If you are unable or prefer not to use a type guard, you can explicitly declare your understanding to the compiler using a type assertion (e.g.
var definitelyFoo = <fooInterface> valueWhen considering whether the value is either a type of fooInterface or string, it is crucial to check the type before proceeding with any operations on the value. In this scenario, you can simply verify if value is of type string using the typeof operator. If not, then it must be of type fooInterface.
interface fooInterface {
bar: any;
}
function(value: fooInterface | string) {
if (typeof value === "string") {
// The compiler now recognizes that the value is a string
}
else {
/* The compiler intelligently deduces that
the value must be of type fooInterface. */
value.bar
}
}
In alternative scenarios, one would need to utilize instanceof (to determine if an object is an instance of a specific class) or create their own custom type checks (especially when dealing with multiple interfaces or custom types).
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