How does initialization work in JavaScript? I am getting an error on the below code saying cannot access obj before initialization.
let a = 7;
let b = 10;
const obj = { a:23, b:35, c:70 }
({a, b} = obj);
console.log(a, b);
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const {a, b} = obj;Mellet– Mellet2021-02-16 17:18:34 +00:00Commented Feb 16, 2021 at 17:18
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@Mellet - OP seems to want to update existing variables.T.J. Crowder– T.J. Crowder2021-02-16 17:19:17 +00:00Commented Feb 16, 2021 at 17:19
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@T.J.Crowder Ah, yes. Sorry ;)Mellet– Mellet2021-02-16 17:19:50 +00:00Commented Feb 16, 2021 at 17:19
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Does this answer your question? What are the rules for JavaScript's automatic semicolon insertion (ASI)?Sebastian Simon– Sebastian Simon2021-08-25 01:56:02 +00:00Commented Aug 25, 2021 at 1:56
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2 Answers
It's because you're relying on Automatic Semicolon Insertion and no automatic semicolon is added after the const obj... line. Without one, that line and the following one are treated as one expression which does indeed try to access obj before it's initialized:
const obj = { a:23, b:35, c:70 }({a, b} = obj);
To the parser, that looks like a function call. If you weren't trying to access obj in what the parser thinks is an arguments list, it would have failed when it got to the point of calling the object, since the object isn't callable.
You need an explicit semicolon to separate the statements:
let a = 7;
let b = 10;
const obj = { a:23, b:35, c:70 }; // <==== Here
({a, b} = obj);
console.log(a, b);Looking at your code, you probably just forgot this one since you include other ones. But if you're going to rely on ASI, make sure you prefix every line starting with (, [, or ` with a ; to prevent that line from continuing the previous expression.
1 Comment
Mukwenha Wolton
T.J. Crowder Thanks a lot it worked. I was relying on ASI so i don't know how it missed it.
Put semicolon after the }
let a = 7;
let b = 10;
const obj = { a:23, b:35, c:70 };
({a, b} = obj);
console.log(a, b);