Mastering TypeScript 3 - Third Edition

Definite assignment

There are occasions, however, where we would like to declare a variable using the let keyword and use it, before TypeScript thinks that it has been defined. Consider the following code:

let globalString: string; 
 
setGlobalString(); 
 
console.log(`globalString : ${globalString}`); 
 
function setGlobalString() { 
    globalString = "this has been set"; 
} 

Here, we have declared a variable named globalString. We then call a function named setGlobalString, which sets the value of the globalString variable. So, according to the logic of this code, the globalString variable is not undefined, and should contain a value at this stage. The next line of code logs the value of globalString to the console.

Unfortunately, the TypeScript compiler ...