'number' is never reassigned. Use 'const' instead. (prefer-const)

Question

Why in this case eslint 4.17.0 i have error number is never reassigned. Use 'const' instead. (prefer-const). Why i need to use const? Please, explain me i can't understand.

let test = {
    'number': 1,
    'string': 'asd',
};
test.number = 99;

console.log(test.number);
// output: 99

ecmascript

 {
    "parser": "babel-eslint",
    "env": {
        "browser": true
    },
    "extends": [
        "google"
    ],
    "rules": {
        "prefer-const": 2

    },
    "parserOptions": {
        "ecmaVersion": 6,
        "sourceType": "module"
    }
}

eslint problem

[eslint] 'test' is never reassigned. Use 'const' insted. (prefer-const)

It's because `test` is *not* changed. You're changing `test.number`, not `test`. — freedomn-m, Feb 09 '18 at 14:38
Also, your error message does not match your question title. One says `test` (correctly) the title says "number" which is changed and is not in the warning. — freedomn-m, Feb 09 '18 at 14:39
Dang, I was looking for this rule. Somehow VSCode stopped suggesting this to me. Looks like it was just this project where I don't have my usual eslint rules ‍♂️. Without eslint it's like missing an eye. — Qwerty, Dec 20 '22 at 22:23

score 5 · Accepted Answer · answered Feb 09 '18 at 13:55

5

ES6 const does not indicate that a value is ‘constant’ or immutable. A const value can definitely change. The following is perfectly valid ES6 code that does not throw an exception.

const foo = {};
foo.bar = 42;
console.log(foo.bar);
// → 42

In your case, if you know that you are gonna change the properties, try using let.

Take a look here: https://mathiasbynens.be/notes/es6-const

answered Feb 09 '18 at 13:55

Cristian S.

937
5
13

1

My mistake the example is with leth, why eslint warning me that need to be const? – Oto_Shan Feb 09 '18 at 14:28
3

@Oto_Shan because `let` assumes that the variable value will be changed at some point of execution which in your example is not happening. – lolbas Feb 09 '18 at 14:30

score 2 · Answer 2 · answered Apr 24 '18 at 09:06

From ESlint Docs:

If a variable is never modified, using the const declaration is better. Const declaration tells readers, “this variable is never modified,” reducing cognitive load and improving maintainability.

        /*eslint prefer-const: 2*/
/*eslint-env es6*/

let a = 3;               /*error 'a' is never modified, use 'const' instead.*/
console.log(a);

// `i` is re-defined (not modified) on each loop step.
for (let i in [1,2,3]) {  /*error 'i' is never modified, use 'const' instead.*/
    console.log(i);
}

// `a` is re-defined (not modified) on each loop step.
for (let a of [1,2,3]) { /*error 'a' is never modified, use 'const' instead.*/
    console.log(a);
}

'number' is never reassigned. Use 'const' instead. (prefer-const)

2 Answers2

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