定义函数作为参数执行

Question

i use destructring function from ES6, i will define function execute as arguments

this is an example

  reformatDate(date: Date) {
    const dd = date.getDate();
    const mm = date.getMonth() + 1;
    const yyyy = date.getFullYear();
    return `${yyyy}-${mm < 10 ? '0' + mm : mm}-${dd < 10 ? '0' + dd : dd}`;
  }

  reformatDate(new Date('2013-3-3'))

with destructuring , we can use

  const reformatDate = ({getDate: dd = dd(), getMonth: mm = mm(), getFullYear:yyyy=yyyy() })  => 
   `${yyyy}-${mm < 10 ? '0' + mm : mm}-${dd < 10 ? '0' + dd : dd}`;

  reformatDate(new Date('2013-3-3'))

 const reformatDate = ({getDate: dd = dd(), getMonth: mm = mm(), getFullYear:yyyy=yyyy() }) => `${yyyy}-${mm < 10 ? '0' + mm : mm}-${dd < 10 ? '0' + dd : dd}`; console.log(reformatDate(new Date('2013-3-3')))

but there are some errors

function getFullYear() { [native code] }-function getMonth() { [native code] }-function getDate() { [native code] }

i used

const reformatDate = ({getDate: dd = dd.call ,....

but same error :( 

Answer 1

The = dd() etc syntax doesn't call the method, it just sets a default parameter. You'd need to call the methods yourself inside the function. However, you can't just call the destructured functions stand-alone, as the this would no longer be bound to a Date object. To do what you want you'd need to retain the original date. One way you could do this is by passing the date in twice to your function, or using an object and destructuring the date property. You'd then need to use .call() on your methods to reassign the this back to your date object like so:

 const reformatDate = ({d, d: {getDate: dd, getMonth: mm , getFullYear:yyyy}}) => `${yyyy.call(d)}-${mm.call(d) < 10 ? '0' + mm.call(d) : mm.call(d)}-${dd.call(d) < 10 ? '0' + dd.call(d) : dd.call(d)}`; console.log(reformatDate({d: new Date('2013-3-3')}))

However, this is essentially defeating the purpose of destructuring in the first place as you could just use these methods on the date object directly. Furthermore, I find it much harder to read and interpret than your first example without the destructuring, and so, I would highly favour your first code-block over this method.

Answer 2

As per docs - MDN ,

Default function parameters allow named parameters to be initialized with default values if no value or undefined is passed.

For example, if your code is like:

 function test(x = 10) { console.log(x); } test(); test(5)

defaulting will be applied if x is not passed.

In your case, since you are using destructing pattern on arguments, defaulting will be applied if the value after destructing is undefined

Following is a sample:

 function test({x = 10, y = 20}) { console.log(x, y); } test({ y: 5 }); test({ x: 4, y: 11 })