What is the term for specifying a variable's data type using a set of values instead of a traditional type?

Recently, I stumbled upon some code that introduces a class with specific variables defined in an unconventional manner.

export class Foo {
  id: string = "A regular string"
  bar: '>' | '<' | '=' | '<=' | '>=' = '>';
}

The variable 'bar' caught my attention as it deviates from the usual type definition style. Instead of specifying a type directly, it consists of a list of strings separated by pipes, followed by an assignment to one of these strings at the end.

My intuition suggests that this setup restricts the possible values for the variable 'bar' to those listed and eventually assigns the value '>'.

Is there a term or name associated with this unique declaration method? I couldn't find any information on it in the official documentation on variable declarations.

Answer №1

The code you're examining represents a concept known as a union type: https://www.typescriptlang.org/docs/handbook/advanced-types.html

Within this structure, the variable bar is constrained to one of the specified values following the colon (>, <, =, <=, or >=)

The final = signifying the default value for bar, in this case being >, as you correctly pointed out.

I trust this explanation was useful. Feel free to refer to the link I provided for further insight into union types.

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