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The JavaScript Pipeline Operator ( |> ) is used to pipe the value of an expression into a function. This operator makes chained functions more readable. This function is called using ( |> ) operator and whatever value is used on the pipeline operator is passed as an argument to the function.
By using the new and currently experimental pipeline operator, Typescript enables you to write such chained function calls from left to right, where the original input is now at the left edge.
The pipe operator, written as %>% , has been a longstanding feature of the magrittr package for R. It takes the output of one function and passes it into another function as an argument. This allows us to link a sequence of analysis steps.
This article opts to use the term type variables, coinciding with the official Typescript documentation. T stands for Type, and is commonly used as the first type variable name when defining generics. But in reality T can be replaced with any valid name.
This is called union type in typescript.
A union type describes a value that can be one of several types.
Pipe (|) is used to separate each type, so for example number | string | boolean is the type of a value that can be a number, a string, or a boolean.
let something: number | string | boolean;
something = 1; // ok
something = '1'; // ok
something = true; // ok
something = {}; // Error: Type '{}' is not assignable to type 'string | number | boolean'
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And here's an example similar to one in the question:
class Test1 {
public a: string
}
class Test2 {
public b: string
}
class Test3 {
}
let x: (typeof Test1 | typeof Test2)[];
x = [Test1]; //ok
x = [Test1, Test2]; //ok
x = [Test3]; //compilation error
x is an array containing constructors of Test1 or Test2.
The pipe represents 'or'. So in this context it says that either of the declared types is allowed. Perhaps it is easy to understand a union with primitive types:
let x: (string | number);
x = 1; //ok
x = 'myString'; //ok
x = true; //compilation error for a boolean
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