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 string type.

Now let's try to add another value of number type 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 string type.

Bonus

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 Audi, BMW, 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 😃!

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