Basic Types Back

TypeScript has supported much the same types of the simplest units of data as we expect in JavaScript, like Number, String, Structure, Boolean, etc. This chapter mainly aims to make note around basic types of TypeScript.

1. Boolean

The most basic data type is the simple true / false value, which both JavaScript and TypeScript call a boolean value:

let isDone: boolean = false;

2. Number

In TypeScript, all numbers are floating point values as same as JavaScript, which get the type, named number. Certainly, decimal, hexadecimal, binary or even octal literals are also supported in TypeScript:

let decimal: number = 6;
let hex: number = 0xff;
let binary: number = 0b1001;
let octal: number = 0o777;

3. String

All textual data are both used the type string to refer to by JavaScript and TypeScript. As in JavaScript, we can also use double quotes or single quotes to surround textual data in TypeScript:

let color: string = '#a10000';

Template Literals has been widely used since ECMAScript 2016, and in TypeScript, we can also use it with embedded expressions:

let str: string = 'Aleen';
console.log(`Hello, ${str}`);

4. Array

Array types is a basic data type for storing data, and in TypeScript, we can declare this type in two ways. One is to use [] literal:

let list: number[] = [1, 2, 3];

And the other is to code like generic programming:

let list: Array<number> = [1, 2, 3];

Tuple is a new feature for TypeScript to declare an array where the type of a fixed number of elements is known. For instance, we can declare an array type with tuple as a pair of a string and a number:

let list: [string, number];
list = ['hello', 10]; /** OK */
list = [10, 'hello']; /** Error */

With tuple specification, accessing operations are also limited as TypeScript has known all types of elements inside this array:

console.log(list[1].substr(1)); /** Error */

What about elements outside the set of known indices? TypeScript will treat them as a union type of string and number:

list[2] = 'world'; /** OK */
console.log(list[3].toString()) /** OK, as both `number` and `string` has the method `toString()` */
list[4] = true; /** Error */

Note: more details of union types can be checked later.

5. Enum

enum is one of the helpful addition to the standard set of data types from JavaScript, which is mainly used to specify a type of set contains numeric values in TypeScript:

enum Color { Red, Green, Blue }
let color: Color = Color.Blue;

Or, we can also specify a specific number for each value:

enum Color { Red = 1, Green = 2, Blue = 4 }

Note: If you don't specify the number, the first item is mapped to zero by default.

TypeScript has also supported a handy feature for us to extract names of an enumerable type with a specific indices:

enum Color { Red = 1, Green, Blue }
let color: string = Color[2]; /** "Green" */

6. Any

Any unknown type can be described as any, like declaring a dynamic variable for accepting different basic types. In such a case, TypeScript won't throw any type error during compiling time:

let notSure: any = 4;
notSure = 'string?'; /** OK */
notSure = true; /** also OK */

Remember that do not expect Object to play a similar role as any, because we can only assign members for it, but not able to access methods.

let notSure: any = 4;
notSure.toFixed(); /** as TypeScript not check */

let notSure: Object = 4;
notSure.toFixed(); /** Error: no such a method, named 'toFixed', in the type `object` */

any type can also used to specify an array, composed with different types:

let list: any[] = [1, true, 'string'];

7. Void, Null and Undefined

As the official document said that, void type was a little like the opposite of any, which we can only assign undefined or null to it:

let unusable: void = void 0;

In most cases, void type is mainly used for specifying result returned from a function:

function warn(): void {
    console.log('only warn');

In TypeScript, null and undefined are both a kind of types as same as JavaScript, which are much like void. They are subtypes of any other type by default, which means we can assign null or undefined to any other type like number, except enabling strictNullChecks options for avoiding a lot of common errors, documented in this note. If we don't ensure whether a specified variable will be assigned with null or undefined, we can use union types to workaround this situation.

8. Never

When there are types of values that never occur, the never type is what we need to specify, which is mainly used as the return type for a function expression:

  • an arrow function expression that always throws an exception
  • a function never returns
function error(message: string): never {
    throw new Error(message); /** returning never has an unreachable end point */

function infiniteLoop() {
    while(true) {}

9. Object

object is a type that represents any non-primitive type, such as number, string, boolean, symbol, null, or undefined.

declare function create(o: object | null): void;

create({ prop: 0 }); /** OK */
create(null); /** OK */

create(1); /** Error */
create('string'); /** Error */
create(false); /** Error */
create(undefined); /** Error */

10. Type Assertions

Type assertions are a way to tell the compiler to trust what developers specify, which is like a type cast in other languages. It has no runtime impact, ans is used purely by the compiler.

There are two types of type assertions, and one is the "angle-bracket" syntax:

let someValue: any = 'string';
let strLength: number = (<string>someValue).length;

The other is the as -syntax:

let someValue: any = 'string';
let strLength: number = (someValue as string).length;

Note: when using TypeScript with JSX, only as -style assertions are allowed as <xxx> will be treated as elements.

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