Typescript 學習筆記 - Section 3

Section Overview

這章節主要會介紹 Type Annotation 跟 Type Inference

Type Annotation and inference

Plain Definition + Overview

Type Annotation - 指的是我們告訴 Typescript 我們宣告的變數 type 是什麼 Type Inference - Typescript 主動去判斷變數的 type 是什麼

Examples

// apple's type is number
const apple: number = 5;
 
// typescript will throw error
let apple: number = 5;
apple = 'asdfdsfds';
 
let speed: string = 'fast';
let hasName: boolean = true;
 
let nothingMuch: null = null;
let nothing: undefined = undefined;
 
// build in object
let now: Date = new Date();
 
// object literal annotations
let colors: string[] = ['red', 'green', 'blue'];
let myNubmers: number[] = [1, 2, 3];
let truths: boolean[] = [true, true, false];
 
// Classes
class Car {}
 
const car: Car = new Car();
 
// object literal
let point: { x: number; y: number } = {
  x: 10,
  y: 20,
};
 
// Function
const logNumber: (i: number) => void = (i: number) => {
  console.log(i);
};
 
// type annotation
// typescript will determine apple's type should be number
let apple = 5;

// const color is variable declaration
// 'red' is variable Initialization
const color = 'red';

如果宣告變數(declaration)以及初始化(initialization)都是在同一行程式碼，Typescript 會自動判別 color 的 tpye 應該要是什麼，所以在一般情況下我們可以使用 type inference 在任何地方，Type annotations 只會用在三種情境

當我們宣告變數跟初始化變數不在同一行程式碼
當我們宣告的變數沒辦法被 inferred
當 function 回傳’any’ type，並且我們需要明確定義 type

The ‘Any’ Type

// When to use annotations
// 1) Functions that return the 'any' type
const json = '{"x": 10, "y": 20}';
const coordinates = JSON.parse(json);
console.log(coordinates); // {x: 10, y: 20}
 
// Typescript will not recognize the error
coordinates.dfgfdsgdfg;

因為 JSON.parse 會回傳的 type 可能是 number, string, object, etc…, 所以 JSON.pars 回傳的 type 是 any any 在 Typescript 也是一種型別，Typescript 並沒有辦法去檢查正確的 property references 在任何情況下應該避免使用’any’

// fix the any type
const json = '{"x": 10, "y": 20}';
const coordinates: { x: number, y: number } = JSON.parse(json);
console.log(coordinates); // {x: 10, y: 20}
 
// Typescript will throw the error
coordinates.dfgfdsgdfg;

Delayed Initialization

// 2) When we declare a variable on one line
// and initializate it later
let words = ['red', 'green', 'blue'];
// change to foundWord: boolean
let foundWord;
 
for (let i = 0; i < words.length; i++) {
  if (word[i] === 'green') {
    foundWord = true;
  }
}

When Inference Doesn’t Work

// 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 (number[i] > 0) {
    // typescript will throw error
    numberAboveZero = numbers[i];
  }
}

這邊可以修正為

let numberAboveZero: boolean | number = false;