JS.BASE.PREFER.CONST

Require 'const' declarations for variables that are never reassigned after declared

If a variable is never reassigned, using the const declaration is better.

const declaration tells readers, "this variable is never reassigned," reducing cognitive load and improving maintainability.

Rule Details

This rule is aimed at flagging variables that are declared using let keyword, but never reassigned after the initial assignment.

Examples of incorrect code for this rule:

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/*eslint prefer-const: "error"*/

// it's initialized and never reassigned.
let a = 3;
console.log(a);

let a;
a = 0;
console.log(a);

class C {
    static {
        let a;
        a = 0;
        console.log(a);
    }
}

// `i` is redefined (not reassigned) on each loop step.
for (let i in [1, 2, 3]) {
    console.log(i);
}

// `a` is redefined (not reassigned) on each loop step.
for (let a of [1, 2, 3]) {
    console.log(a);
}

Examples of correct code for this rule:

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/*eslint prefer-const: "error"*/

// using const.
const a = 0;

// it's never initialized.
let a;
console.log(a);

// it's reassigned after initialized.
let a;
a = 0;
a = 1;
console.log(a);

// it's initialized in a different block from the declaration.
let a;
if (true) {
    a = 0;
}
console.log(a);

// it's initialized in a different scope.
let a;
class C {
    #x;
    static {
        a = obj => obj.#x;
    }
}

// it's initialized at a place that we cannot write a variable declaration.
let a;
if (true) a = 0;
console.log(a);

// `i` gets a new binding each iteration
for (const i in [1, 2, 3]) {
  console.log(i);
}

// `a` gets a new binding each iteration
for (const a of [1, 2, 3]) {
  console.log(a);
}

// `end` is never reassigned, but we cannot separate the declarations without modifying the scope.
for (let i = 0, end = 10; i < end; ++i) {
    console.log(a);
}

// `predicate` is only assigned once but cannot be separately declared as `const`
let predicate;
[object.type, predicate] = foo();

// `a` is only assigned once but cannot be separately declared as `const`
let a;
const b = {};
({ a, c: b.c } = func());

// suggest to use `no-var` rule.
var b = 3;
console.log(b);

Options

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{
    "prefer-const": ["error", {
        "destructuring": "any",
        "ignoreReadBeforeAssign": false
    }]
}

destructuring

The kind of the way to address variables in destructuring. There are 2 values:

"any" (default) - If any variables in destructuring should be const, this rule warns for those variables.
"all" - If all variables in destructuring should be const, this rule warns the variables. Otherwise, ignores them.

Examples of incorrect code for the default {"destructuring": "any"} option:

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/*eslint prefer-const: "error"*/
/*eslint-env es6*/

let {a, b} = obj;    /*error 'b' is never reassigned, use 'const' instead.*/
a = a + 1;

Examples of correct code for the default {"destructuring": "any"} option:

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/*eslint prefer-const: "error"*/
/*eslint-env es6*/

// using const.
const {a: a0, b} = obj;
const a = a0 + 1;

// all variables are reassigned.
let {a, b} = obj;
a = a + 1;
b = b + 1;

Examples of incorrect code for the {"destructuring": "all"} option:

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/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
/*eslint-env es6*/

// all of `a` and `b` should be const, so those are warned.
let {a, b} = obj;    /*error 'a' is never reassigned, use 'const' instead.
                             'b' is never reassigned, use 'const' instead.*/

Examples of correct code for the {"destructuring": "all"} option:

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/*eslint prefer-const: ["error", {"destructuring": "all"}]*/
/*eslint-env es6*/

// 'b' is never reassigned, but all of `a` and `b` should not be const, so those are ignored.
let {a, b} = obj;
a = a + 1;

ignoreReadBeforeAssign

This is an option to avoid conflicting with no-use-before-define rule (without "nofunc" option). If true is specified, this rule will ignore variables that are read between the declaration and the first assignment. Default is false.

Examples of correct code for the {"ignoreReadBeforeAssign": true} option:

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/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": true}]*/
/*eslint-env es6*/

let timer;
function initialize() {
    if (foo()) {
        clearInterval(timer);
    }
}
timer = setInterval(initialize, 100);

Examples of correct code for the default {"ignoreReadBeforeAssign": false} option:

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/*eslint prefer-const: ["error", {"ignoreReadBeforeAssign": false}]*/
/*eslint-env es6*/

const timer = setInterval(initialize, 100);
function initialize() {
    if (foo()) {
        clearInterval(timer);
    }
}

When Not To Use It

If you don't want to be notified about variables that are never reassigned after initial assignment, you can safely disable this rule.