ES6 deconstruction assignment of array of deconstruction assignment

Deconstruct assignment

< One > The deconstruction assignment is a JavaScript Expression, which makes it possible to extract values from an array , Or extract attributes from objects into different variables . ----MDN
< Two > Here's the explanation from the rookie tutorial

1. Deconstructing assignment is an extension of assignment operator .
2. It's a pattern match for an array or an object , And then assign values to the variables .
3. Simple and easy to read in code writing , The meaning is clearer ; It is also convenient to obtain data fields in complex objects .

The following are the contents related to the deconstruction and assignment of some arrays

1. Simple deconstruction assignment

grammar const[ ] = [ ];
The array to the left of the equal sign is the variable to be assigned , The array on the right is the array to be deconstructed , Here's the code
const arr = [1,2,3,4];
let [a,b,c,d] = arr;
By this time a = 1; b=2; c=3; d= 4;

2. More complex deconstruction assignment

If we define a more complex array like the following line of code, how can we get the corresponding data

const arr = ['a','b',['c','d',['e','f','g']]];

Here are some points we can remember , namely

1. Can be nested
2. Negligible
3. Incomplete deconstruction
4. Remainder operator
5. The default value is

For example, we need to get... In the last array b, We can do this

const arr = ['a','b',['c','d',['e','f','g']]];
const [,b] = arr;

We got... In the last array g, We can do this

const arr = ['a','b',['c','d',['e','f','g']]];
const [,,[,,[,,g]]]=arr;

Remainder operator , The following code uses the residual operator , Also called extension operator
What we should pay attention to here is , The remaining elements will be connected together in the form of an array in the original order
Operator must be placed last , Otherwise you will report an error

const arr = ['a','b',['c','d',['e','f','g']]];
const [a,...b] = arr;

Calculation results , By this time b

About deconstructing default values
1. When we deconstruct and assign values to an array , If the number of variables is more than the number of original array elements , The extra variables are directly copied as undefined
2. When the corresponding value of the variable is undfined The default value can be redefined when
for example
const arr = [1, undefined, undefined];
const [a, b = 2, c, d=“aaa”] = arr;
Now a,b,c,d The corresponding is 1,2,undefined,“aaa”;

Incomplete deconstruction

let [a = 1, b] = []; // a = 1, b = undefined