Such as 2[a], whose value is undefined.
The code bellow will get an error Cannot access 'b' before initialization
let a = 1, b = 2
[a, b] = [b, a];
I know it caused by the missing semicolon after the b = 2. And after the semicolon has been added
let a = 1, b = 2;
[a, b] = [b, a];
It works fine. Is it caused by the square bracket after the number? If so, what is the meaning of it in javascript?
Is it caused by the square bracket after the number?
Yes.
If so, what is the meaning of it in javascript?
It's a property access operation, just like:
const array = [1, 2, 3];
const b = array[1];
// ^^^−−−−−−−−−−−−−−−−−−−−−−−−−
console.log(b); // 2You can do property access operations on primitives, and in fact you often do. For instance, on a string primitive:
console.log("hi".toUpperCase()); // "HI"
// or
const fiftyFifty = Math.random() < 0.5;
const method = fiftyFifty ? "toUpperCase" : "toLowerCase";
console.log("Hi"[method]()); // "hi" or "HI"Or using a number primitive:
console.log(2["toString"]()); // "2"
const n = 2;
console.log(n.toString()); // "2"
console.log(2..toString()); // "2"When you do, the relevant prototype is used (String.prototype, Number.prototype, etc.) and in loose mode in some situations a temporary object is created (e.g., new String("hi") or new Number(2)).
