TypeScript学习-第二节(Variable Declar

let

const

Destructuring

• array descructuring assignment

let input = [1, 2]
let [ first, second ] = input;

• already-declared variables

We assume first, second has already declared, and then,
[first, second] = [second, first];
// swap two variable

• parameters to function:

  function f([first, second]: [number, number]) {}
  f([1, 2])

• spread syntax

  let [first] = [1, 2, 3] 
// 1
  let [, first, second] = [1, 2, 3]
// 2, 3

• object destructuring

// a,b 的申明和变量赋值是一起的
//eg. let a; let { a } = o; 报错，因为a已经申明过了
  let { a, b } = o; // o is an object
  ({ a, b } = { a: 1, b: 2 }) // use () means parentheses

• property renaming

//alias name:
  let { a: newName1, b: newName2 } = o;
//type-checking
  let { a, b } : { a: string, b: number } = o;

• Default values

function f(wholeObject: { a: string, b?: number } ) {}

• params destructuring

type C = { a: string, b?: number};
function f({a, b}: C):void {}
function f({a, b = 0} : C = { a: "" }): void {}
f({a: "hhh"}) // ok
f() // ok
f({}) //wrong

spread operator (opposite of destructuring)

（1）It allows you spread an array into another array, an object into another object
（2）array or object not changed by the spread, only creates a shallow copy
（3）Limits
  （1）only 'own', 'enumerable' property
  （2）Doesn't allow spreads of type parameters in generic function