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"
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
Explict 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.
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 a 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.
