Type Annotations and Type inference
Definition
Type annotations -> Code we add to tell Typescript what type of value a variable will refer to.
Type inference -> Typescript tries to figure out what type ofvalue a variable refers to.
Type annotations: We (developers) tell Typescript the type <—>Type inference:Typescript guesses the type
Annotation Examples
------------------------------
// Annotations
// with variables
let apples: number = 5;
let speed: string = 'fast';
let hasName: boolean = true;
let nothingMuch: null = null;
let nothing: undefined = undefined;
// built in objects
let now: Date = new Date();
// Array
let colors: string [] = ['red', 'green', 'blue'];
let myNumbers: number [] =[1,2,3,4];
let truths: boolean [] = [false, true, false];
// Classes
class Car {
}
let car: Car = new Car();
// Object literal
let point: {x: number; y: number} = {
x: 10,
y: 20
}
// Function
// we expect a value of i which is a number and a return value of void (which indicates that we don't want to return anything)
const logNumber: (i: number)=>void = (i: number) =>{
console.log(i);
}
------------------------------
Inference Examples
When to use what?
// When to use annotations
// 1) Function that returns the 'any' type
const json = '{"x": 10, "y": 20 }';
const coordinates = JSON.parse(json);
console.log(coordinates); // {x:10, y: 20};
// --> Fix Any Type: Assing annotation to coordinates
const coordinates: { x: number; y: number } = JSON.parse(json);
// 2) When we declare a variable on one line and initialize it later
let words = ['red', 'green', 'blue'];
let foundWord;
for (let i= 0; i< words.length; i++){#
if (words[i] === 'green'){
foundWord = true;
}
}
// --> Fix: Assing annotation to foundWord
let foundWord: boolean;
// 3)Variable whose type cannot be inferred correctly
let numbers = [-10, -1, 12];
let numberAboveZero = false;
for (let i = 0 ; i< numbers.length; i++){
if (numbers[i]>0){
numberAboveZero =numbers[i];
}
}
// --> Fix: Add annotation with an "or" statement
let numberAboveZero: boolean | number = false;
