Writing less verbose code with TypeScript's type inference

Static typing has a number of benefits; however, it makes us write a more verbose code by adding all the type annotations.

In some cases, the TypeScript's compiler is able to guess the types of expressions inside our code; let's consider this example, for instance:

let answer = 42; 
answer = '42'; // Type "string" is not assignable to type "number" 

In the preceding example, we defined a variable answer and assigned the value 42 to it. Since TypeScript is statically typed and the type of a variable cannot change once declared, the compiler is smart enough to guess that the type of answer is number.

If we don't assign a value to a variable within its definition, the compiler will set its type to any:

let answer; 
answer = 42; 
answer = '42'; 

The preceding snippet will compile without any compile-time errors.

Best common type

Sometimes, the type inference could be a result of several expressions. Such is the case...