How to add a string array type annotation to a variable or a constant in TypeScript?
Published September 5, 2021
To add a
string array type annotation to a variable or a constant, you can declare the variable name followed by a
: (colon) symbol, then the
string type with
 (opening and closing square brackets) symbol.
For example, let's say we have a variable called
names and want to only have the
string type as the values in the array. To do that we can annotate the
string array type to the
names variable like this,
// add string array type to a variable let names: string;
After initialization, let's add a string value of
John Doe to the
names array. It can be done like this,
// add string array type to a variable let names: string; // add a string value to the names array names = ["John Doe"]; // ✅ allowed
As you can see from the above code TypeScript allowed us to add the string value of
John Doe to the string type array
names since the allowed type for the
names array is a
Now let's try to add another value of
1 to the
names array to see what happens,
// add string array type to a variable let names: string; // add a string value to the names array names = ["John Doe"]; // ✅ allowed // add a value of type other than string names = ["John Doe", 1]; // ❌ not allowed. Type 'number' is not assignable to type 'string'.
You can see that TypeScript will throw an error saying
'Type 'number' is not assignable to type 'string', 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
cars and add
string values like
Benz without actually declaring the string array type annotation as we did above. It can be done like this,
// Typescript infers the type from // the value it is assigned let cars = ["Audi", "BMW", "Benz"];
As you can see when you hover over the variable
cars, TypeScript has automatically detected that the variable
cars is a
string array type.
Since the TypeScript has already got the type is of
string array type and whenever we try to reassign a value other than the
string array type, The compiler will throw errors like from the above.
See the execution of the above codes live in codesandbox.
That's all 😃!