function retrieveCounter(): Counter {
let counter = <Counter>function (start: number) { };
counter.interval = 123;
return counter;
}
Upon inspection of line 2 in the code snippet above, I am left wondering why I am unable to use
function (start: number): CounterMy query pertains to understanding the distinction between <type> and :type.
:type is used to declare the return value type of a function. For instance:
let counter = function (start: number): number {}
let result = counter(1);
In this example, the result variable will have a type of number.
Visit function types for further explanation.
<type> signifies a type assertion, indicating to the compiler that the value following this assertion is of the specified type. For your situation, the variable counter will be of type Counter.
If you want to delve into the world of TypeScript, be sure to check out this page from the TypeScript handbook. The section on Type assertions covers the <Type> syntax, which is key to understanding variable types in TypeScript.
The :Type syntax indicates the type of a variable, for example:
let example: string = "This is a string";
let exampleNumber: number = 12;
However, TypeScript is smart and can often infer types without explicit declarations, as shown in this simplified version:
let example = "This is a string";
let exampleNumber = 12;
At times, TypeScript may default to an any type, allowing for flexibility in variable typing. By utilizing both the and :Type syntax, you can enforce strong typing, like so:
function test(arg: any): number {
// Treat arg as a string;
let example: string = arg;
// Treat arg as a string through Type assertion.
let otherExample = <string> arg;
// Returns arg as a number to enforce strong typing.
return arg;
}
// Nr will be inferred as a number by the compiler.
let nr = test(12);
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