TypeError when swapping object properties with destructuring assignment [duplicate]

Question

I am trying to swap two object values in JavaScript using the [] = [] method, but my below code fails with an error saying "message": "Uncaught TypeError: Cannot set property '9' of undefined",

let dataObj={"reg_price":2, "reg_price_alt":5, "ex":9}
console.log("before: ", dataObj)
[dataObj.reg_price, dataObj.ex] = [4, 5];
console.log("after: ", dataObj)

Is there some syntax I'm missing? I don't understand why this simple code does not work.

I disagree because I would like to complete this task on a single line if possible, which is why for swapping two variables it would be simple to just use the line: [dataObj.reg_price, dataObj.ex] = [dataObj.ex, dataObj.reg_price]; which works with variables not in an object — Martin
– Martin, Commented Mar 22, 2021 at 21:13
In destructuring assignment, the targets in the array have to be simple variable references. So you can't put object property references there. — Barmar
– Barmar, Commented Mar 22, 2021 at 21:17
@Teemu I've tried using deconstructing syntax like ({ dataObj.reg_price, dataObj.ex } = { dataObj.reg_price: 10, dataObj.ex: 20 }); but it doesnt work — Martin
– Martin, Commented Mar 22, 2021 at 21:17
Wait, this syntax works fine. You're just missing a semicolon. — ggorlen
– ggorlen, Commented Mar 22, 2021 at 21:21

ggorlen · Accepted Answer · 2021-03-22 21:33:45Z

7

The syntax is fine. Add a semicolon to prevent the automatic semicolon insertion from thinking you want to do console.log(...)[...] instead of array destructuring:

let dataObj = {"reg_price":2, "reg_price_alt":5, "ex":9}
console.log("before: ", dataObj); // <-- semicolon
[dataObj.reg_price, dataObj.ex] = [4, 5]
console.log("after: ", dataObj)

I'd take this a step further and add semicolons after every line. Caveat emptor otherwise. Example of swapping values:

const o = {a: 0, b: 1};
console.log(o);
[o.a, o.b] = [o.b, o.a];
console.log(o);

edited Mar 22, 2021 at 21:33

answered Mar 22, 2021 at 21:18

ggorlen

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