Object 解构：在一行中分配 null 个值

Question

Assuming I have an object like this

foo = {
    a: 1,
    b: 2,
    c: 3
};

I would destructure it this way

const {a, b, c} = foo;

However, what, if the keys a , b and c do not exist?

foo = null;

This won't work then

const {a, b, c} = foo;

I would have to do something like

let a, b, c;

if(foo) {
    a = foo.a;
    b = foo.b;
    c = foo.c;
};

Is there a way to do this in one line? Something like

 const {a, b, c} = foo ? foo : null; // this is not working

null is not an object. (evaluating '_ref2.a')

Answer 1

When foo is not an object, you are going to have to give it an object so the destructuring can happen. So set it to an empty object using an 'or'. Now if foo is defined it will use foo , if it is not defined it will use the default object.

 const foo = undefined; const { a, b, c } = foo || {}; console.log(a, b, c);

If you want default values, set them in the object.

 const foo = undefined; const { a, b, c } = foo || {a: 'a', b: 'b', c: 'c' }; console.log(a, b, c);

If you want default values if the object does not provide them all, you can set values in the destructuring.

 const foo = { a: 1}; const { a = "a", b = "b", c = "c" } = foo || {}; console.log(a, b, c);

Answer 2

What about destructuring default parameters? If foo could be undefined set = foo || {} = foo || {} in destructuring to avoid exception

 let foo = {}; const {a = null, b = null, c = null} = foo; console.log('a:', a); console.log('b:', b); console.log('c:', c);

Answer 3

It will work like you might expect, the variables a b and c will be created with the value of undefined . this is the fallback when also trying to access a property which doesn't exist on a certain object.

 const foo = {}; console.log(foo.a); // undefined const {a, b, c} = foo; console.log(a); // undefined console.log(b); // undefined console.log(d); // Uncaught ReferenceError: d is not defined