How to add a boolean array type annotation to a variable or a constant in TypeScript?
Published September 7, 2021
To add a
boolean array type annotation to a variable or a constant, you can declare the variable name followed by a
: (colon) symbol, then the
boolean type with
 (opening and closing square brackets) symbol.
For example, let's say we have a variable called
bools and want to only have the
boolean type as the values in the array. To do that we can annotate the
boolean array type to the
bools variable like this,
// add boolean array type to a variable let bools: boolean;
After initialization, let's add a boolean value of
true to the
bools array. It can be done like this,
// add boolean array type to a variable let bools: boolean; // add a boolean value to the bools array bools = [true]; // ✅ allowed
As you can see from the above code TypeScript allowed us to add the boolean value of
true to the boolean type array
bools since the allowed type for the
bools array is a
Now let's try to add another value of
Hello World! to the
bools array to see what happens,
// add boolean array type to a variable let bools: boolean; // add a boolean value to the bools array bools = [true]; // ✅ allowed // add a value of type other than boolean bools = [true, "Hello World!"]; // ❌ not allowed. Type 'string' is not assignable to type 'boolean'.
You can see that TypeScript will throw an error saying
'Type 'string' is not assignable to type 'boolean', which is also what we want.
Also, we cannot use the
push() method to add a value other than the
When a variable is assigned a value, TypeScript will attempt to determine the type of the variable.
For example, let's declare a variable called
favBools and add
boolean values like
true without actually declaring the boolean array type annotation as we did above. It can be done like this,
// Typescript infers the type from // the value it is assigned let favBools = [true, false, true];
As you can see when you hover over the variable
favBools, TypeScript has automatically detected that the variable
favBools is a
boolean array type.
Since the TypeScript has already got the type is of
boolean array type and whenever we try to reassign a value other than the
boolean array type, The compiler will throw errors like from the above.
See the execution of the above codes live in codesandbox.
That's all 😃!