Mastering TypeScript 3 - Third Edition

Type syntax

The TypeScript syntax for declaring the type of a variable is to include a colon (:) after the variable name, and then indicate its type. As an example of this, let's rewrite our problematic doCalculation function to only accept numbers. Consider the following TypeScript code:

function doCalculation( 
    a : number, 
    b : number, 
    c : number) { 
    return ( a * b ) + c; 
} 
 
var result = doCalculation(3,2,1); 
console.log("doCalculation():" + result); 

Here, we have specified that the doCalculation function needs to be invoked with three numbers. This means that the properties a, b, and c must be of type number in order to call this function. If we now attempt to call this function with strings, as we did with the JavaScript sample, as follows: ...