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TypeScript Simple Types

TypeScript supports some simple types (primitives) you may know.

There are three main primitives in JavaScript and TypeScript.

  • boolean - true or false values
  • number - whole numbers and floating point values
  • string - text values like "TypeScript Rocks"

Type Assignment

When creating a variable, there are two main ways TypeScript assigns a type:

  • Explicit
  • Implicit

In both examples below firstName is of type string

Explicit Type

Explicit - writing out the type:

let firstName: string = "Dylan";
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Explicit type assignment are easier to read and more intentional.

Implicit Type

Implicit - TypeScript will "guess" the type, based on the assigned value:

let firstName = "Dylan";
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Note: Having TypeScript "guess" the type of a value is called infer.

Implicit assignment forces TypeScript to infer the value.

Implicit type assignment are shorter, faster to type, and often used when developing and testing.

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Error In Type Assignment

TypeScript will throw an error if data types do not match.


let firstName: string = "Dylan"; // type string
firstName = 33; // attempts to re-assign the value to a different type
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Implicit type assignment would have made firstName less noticeable as a string, but both will throw an error:


let firstName = "Dylan"; // inferred to type string
firstName = 33; // attempts to re-assign the value to a different type
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JavaScript will not throw an error for mismatched types.

Unable to Infer

TypeScript may not always properly infer what the type of a variable may be. In such cases, it will set the type to any which disables type checking.


// Implicit any as JSON.parse doesn't know what type of data it returns so it can be "any" thing...
const json = JSON.parse("55");
// Most expect json to be an object, but it can be a string or a number like this example
console.log(typeof json);
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This behavior can be disabled by enabling noImplicitAny as an option in a TypeScript's project tsconfig.json. That is a JSON config file for customizing how some of TypeScript behaves.

Note: you may see primitive types capitalized like Boolean.

boolean !== Boolean
For this tutorial just know to use the lower-cased values, the upper-case ones are for very specific circumstances.

TypeScript Exercises

Test Yourself With Exercises


There are two main ways TypeScript assigns a type:


Start the Exercise