How to add a number type annotation to a variable or a constant in TypeScript?

Published September 3, 2021

To add a number type annotation to a variable or a constant, you can declare the variable or constant followed by a : (colon) symbol and then the type number in TypeScript.

For example, let's say we have a variable called myNumber and want to only have the number type as the values. To do that we can annotate the type number to the myNumber variable like this,

// add number type to a variable
let myNumber: number;

After initialization, let's assign a number value of 100 to the variable myNumber. It can be done like this,

// add number type to a variable
let myNumber: number;

// assign a number value to the myNumber variable
myNumber = 100; // ✅ allowed

As you can see from the above code TypeScript allowed us to assign the number value of 100 to the variable myNumber since the allowed type for the myNumber variable is number.

Now let's try to assign another value of string type Hello World! to the myNumber variable to see what happens,

// add number type to a variable
let myNumber: number;

// assign a number value to the myNumber variable
myNumber = 100; // ✅ allowed

// assign a value of type other than a number
myNumber = "Hello World!"; // ❌ not allowed. Type 'string' is not assignable to type 'number'.

As you can see that the TypeScript will throw an error saying that the Type 'string' is not assignable to type 'number'. which is also what we want to happen.

Bonus

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 myFavNum and assign the number value of '1000' without actually declaring the type number as we did above. It can be done like this,

// Typescript infers the type from
// the value it is assigned
let myFavNum = 1000;

Now if you hover over the variable myFavNum we can see that TypeScript has automatically got that type of the variable myFavNum is of type number.

Since the TypeScript has already got the type is of number value and whenever we try to reassign a value other than the type number, 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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