What is happening when you're destructing an array without using square brackets?

Question

Python developer here. I am using JavaScript to run the following command,

let a = [1, 2, 3];
let b , c, d = a;
console.log(b);
console.log(c);
console.log(d);

To my surprise, b and c are undefined while d is the entire a variable. I later realize that I am meant to go let [b, c, d] = a;, however I am just trying to understand what is happening and why they're undefined.

Answer 1

你不是在解构。

let b , c, d = a;

声明 3 个变量并为其中之一赋值。上面的行与以下内容相同：

let b;
let c;
let d = a;

此语法允许您在单个语句中声明（并可选择分配）多个变量，例如

let tmp, x = 4, i, y = 2, z = x * 10 + y;

未分配的变量默认初始化为undefined。

参考：

• ECMAScript 2024：14.3.1 Let 和 Const 声明