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TypeScript supports some simple types (primitives) you may know
When creating a variable, there are two main ways TypeScript assigns a type:
In both examples below firstName is of type string
let firstName: string = "Dylan";
let firstName = "Dylan";
Note: Having TypeScript "guess" the type of a value is called infer.
Implicit assignment forces TypeScript to infer the value.
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
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
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);
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.