References Back

1. Use const

• To use const for all of your references, and avoid using var.
• Eslint rules tags: prefer-const, no-const-assign

the reason is to ensure that you cannot reassign(重新訪問) references, which can lead to bugs and difficult to comprehend(理解) code.

/**
 * bad 
 */
var a = 1;
var b = 2;

/**
 * good
 */
const a = 1;
const b = 2;

2. Use let

• If you must reassign references, you can use let instead of var.
• Eslint rules tags: no-var

/**
 * bad
 */
var count = 1;
count = count? count + 1 : count;

/**
 * good
 */
let count = 1;
count = count? count + 1 : count;

• Notice that: both const and let are block-scoped(塊作用域).

{
    let a = 1;
    const b = 1;
}
console.log(a); /** ReferenceError  */
console.log(b); /** ReferenceError  */