Union of Types in TypeScript

It happens that a variable can take values of different types. You already know that in this case you can declare this variable with the type any.

However, there are situations when we know that a variable can take values of not all types, but only some. For example, it can be either a string or a number.

In this case, it would be better to allow the variable to accept only the required types. This is done using union operator of types, representing a vertical stick.

Let's use this operator to allow a variable to be either a string or a number:

let test: string | number;

Let's check - write a number into a variable:

test = 123;

Now let's write the following line into it:

test = 'abc';

Now let's write down the logical value:

test = true; // there will be an error

Make a variable that can be either a number or null.

Make a variable that can be either a number, a string, or a boolean.