How to add a boolean type annotation to a variable or a constant in TypeScript?
Published September 4, 2021
To add a boolean type annotation to a variable or a constant, you can declare the variable or constant followed by a
: (colon) symbol and then the type
boolean in TypeScript.
For example, let's say we have a variable called
isPerson and want to only have the
boolean type as the values. To do that we can annotate the type
boolean to the
isPerson variable like this,
// add boolean type to a variable let isPerson: boolean;
After initialization, let's assign a boolean value of
true to the variable
isPerson. It can be done like this,
// add boolean type to a variable let isPerson: boolean; // assign a boolean value to the isPerson variable isPerson = true; // ✅ allowed
As you can see from the above code TypeScript allowed us to assign the boolean value of
true to the variable
isPerson since the allowed type for the
isPerson variable is
Now let's try to assign another value of
Hello World! to the
isPerson variable to see what happens,
// add boolean type to a variable let isPerson: boolean; // assign a boolean value to the isPerson variable isPerson = true; // ✅ allowed // assign a value of type other than a boolean isPerson = "Hello World!"; // ❌ not allowed. Type 'string' is not assignable to type 'boolean'.
As you can see that the TypeScript will throw an error saying that the
Type 'string' is not assignable to type 'boolean'. which is also what we want to happen.
Most of the time TypeScript will try to infer the type of the variable from the value it is assigned.
For example, let's declare a variable called
myFavBool and assign the
boolean value of 'true' without actually declaring the type
boolean as we did above. It can be done like this,
// Typescript infers the type from // the value it is assigned let myFavBool = true;
Now if you hover over the variable
myFavBool we can see that TypeScript has automatically got that type of the variable
myFavBool is of type
Since the TypeScript has already got the type is of
boolean value and whenever we try to reassign a value other than the type
boolean, The compiler will throw errors like from the above.
See the execution of the above codes live in codesandbox.
That's all 😃!