1. Block -level scope

1.let and const replaced VAR

The LET declaration variable will not be hung in Window, and will not cause global pollution; let new add -level agglomeration scope; let is not allowed to repeat declarations; let does not have a statement improvement, but it is improved to a temporary dead area.

CONST is the same as Let, but Const defines constants, not allowed to be modified, not allowed to change the memory address, and the Const declaration and assignment must be completed at one time.

console.log(a);
var a = 10;

console.log(b);
let b = 10;

The above code A will output UNDEFINED, B will directly report an error.

In the global environment, the use of CONST, especially in the global environment, should not be set up variables, but only constants should be set. Const declares that there are two benefits. One is that people who read the code immediately realize that this value should not be modified, and the other is to prevent errors caused by inadvertently modifying the value value. All functions should be set to constant.

2. String

Static string uses single quotes or anti -quotes, and dynamic strings use anti -attract numbers.

// bad
const a = "foobar";
const b = 'foo' + a + 'bar';

// acceptable
const c = `foobar`;

// good
const a = 'foobar';
const b = `foo${a}bar`;

3. Array

When using array members to assign variables, the deconstruction assignment is preferred.

const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;

Use the extension operator (...) to copy and conversion to the array

The main role of the extended operator, expand the array, collect the array content

const arrCopy = [...arr];

 let arr = [...document.querySelectorAll('div')]
 console.log(arr)