In this blog post, we will see how can we work with the variables in Typescript.

This is part of the Typescript Tutorial Series, whose index / TOC is available here → Typescript Tutorial Series.

Defining variables

We use the let keyword to define a variable in Typescript. As indicated the value of the variable is NOT fixed (hence the name) and can be changed at any point in time.

The fundamental difference between the Typescript its predecessor Javascript is, Typescript has a strict type checking to ensure there are no gaps on the potential values that can be assigned to variable of a type. For example, we can’t assign a character/alphabet value to a variable which accepts/expects a numerical data - example age.

Advantage of type checking

If such kind of data type mismatch errros are not caught earlier (during the compilation time), they may blow up during the execution of the source code in the runtime, which is certainly recommended as it will disappoint the actual users of the application.

Hence, this static type checking is much preferred and worth in languages like Typescript which is not the case with Javascript, where the users are expected to be diligent enough at all the times.


The syntax to define a variable goes as follows.

let <varName> : <dataType> = <initialValue>;


  • let is a keyword
  • varName is the name of the variable
  • dataType is one of the valid types that can be assigned to a variable in Typescript
  • initialValue is the potentially allowed values according to the datatype
  • ; is a line terminator

The Typescritp datatypes are discussed in this post → Typescript Datatypes

Example 1 - Proper data assignment

let age : number = 21;


The above program is legally valid as it follows the norms, by assiging a numeric data to the variable age whose type is defined as number.

Example 2 - Invalid data assignment

let age : number = 'A';


First of you all, the IDE (VSCode, Eclipse, or any others which has got the tsc embedded) will show a warning in the source code with an icon or bulb indicating that error.

If you still go ahead and compile (transpile) the program, the tsc will throw an error, as follows.

typescriptPractices > tsc variables.ts
variables.ts:1:5 - error TS2322: Type 'string' is not assignable to type 'number'.

1 let age : number = 'A';

Found 1 error.

typescriptPractices >

The error message is pretty clear and self-explanatory.

Note: There is a caveat in this step - as the tsc will still produce the .ts file despite the compiler errors. We discuss how to avoid this in the post.

Example Program with a few different type of variables

The below program demonstrates the usage of the primitive type variables with the valid values.

Source : variables.ts

let found : boolean = true;
let mark : number = 99.1; 
let firstName : string = 'Raghavan'; //surrounded by a pair of single quotes
let lastName : string = "Muthu";    //surrounded by a pair of double quotes


Let us verify the file status.

typescriptPractices > ls -l
total 4
-rw-rw-rw-  1 raghs 0 222 2021-06-19 21:48 variables.ts

typescriptPractices >

We transpile the program using tsc. The ls command confirms the availability of .js file as a result of transpilation by the Typescript Compiler tsc.

typescriptPractices > tsc variables.ts

typescriptPractices > ls -l
total 8
-rw-rw-rw-  1 raghs 0 183 2021-06-19 21:49 variables.js
-rw-rw-rw-  1 raghs 0 222 2021-06-19 21:48 variables.ts

Then, execute the program using the node runtime - node with the generated/transpiled .js file.

typescriptPractices > node variables.js

Raghavan alias Saravanan Muthu
19 Jun 2021 | Sat | 21 50 25 PM IST