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 valuesnumber- whole numbers and floating point valuesstring- text values like "TypeScript Rocks"
There are also 2 less common primitives used in later versions of Javascript and TypeScript.
bigint- whole numbers and floating point values, but allows larger negative and positive numbers than thenumbertype.symbolare used to create a globally unique identifier.
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";
Try it Yourself »
Explicit type assignments 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 value is called infer.
Implicit assignment forces TypeScript to infer the value.
Implicit type assignments are shorter, faster to type, and often used when developing and testing.
Error In Type Assignment
TypeScript will throw an error if data types do not match.
Example
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:
Example
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 variable may be. In such cases, it will set the type to any which disables type checking.
Example
// 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.
