const testOptions: "Undecided" | "Yes" | "No" = "Undecided";
When types are separated by the pipe symbol |, they are referred to as union-types and can be interpreted as an OR operation. The use of the = sign here indicates an assignment, indicating that the property testOptions is initially set to "Undecided".
The code can be restructured in the following way:
// In this example, Foo is specifically designed to only accept string values
// "Undecided", "Yes", or "No". Any other string will not match the specified type.
type Foo = "Undecided" | "Yes" | "No";
// This line will result in an error because "bar" does not fall within the allowed values.
const a: Foo = "bar";
// On the contrary, this declaration will be valid.
const b: Foo = "Undecided";
If you're interested in exploring more complex types in TypeScript, I suggest checking out the documentation on advanced types
When working with typescript, the type of a variable is specified as variableName: type.
The type can vary widely.
An important concept is the union operator (|) which allows for combining multiple types into one.
For example, if a variable can be either a number or a string, it would be defined as variable: number | string.
But what if you want to restrict a variable to specific values within a certain type?
public testOptions: "Undecided" | "Yes" | "No" = "Undecided";
Now, the testOptions variable can only hold values of "Undecided" | "Yes" | "No", with a default value of "Undecided".
Think of it as a simplified version of an enum...
The code below will successfully compile:
testOptions: "Undecided" | "Yes" | "No" = "Undecided";
testOptions: "Undecided" | "Yes" | "No" = "Yes";
testOptions: "Undecided" | "Yes" | "No" = "No";
testOptions: "a" | "b" | "c" = "a";
However, this snippet will not compile:
testOptions: "Undecided" | "Yes" | "No" = "xxx";
testOptions: "a" | "b" | "c" = "xxx";
This is because "xxx" is not included in the specified type declaration.
Contrasted with a traditional enum:
enum MyEnum{
Undecided= 1,
Yes,
No
}
testOptions:MyEnum = MyEnum.Yes; <-- much clearer and straightforward to interpret
public testOptions: "Undecided" | "Yes" | "No" = "Undecided";
"Undecided" | "Yes" | "No".If you try to change the value to
public testOptions: "Undecided" | "Yes" | "No" = "decided";
An error will occur because the value should be one of "Undecided" | "Yes" | "No" instead of a regular string
This code introduces a custom data type in addition to string, object, and number; it defines a new type called
type UndecideYesNoType = "Undecided" | "Yes" | "No";
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