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no-let

Disallow mutable let declarations.

Targeted pattern scope​

This rule targets VariableDeclaration nodes where kind === "let".

What this rule reports​

  • Any let declaration not ignored by configured patterns

Why this rule exists​

const bindings better communicate immutability intent and prevent accidental reassignment.

❌ Incorrect​

let total = 0;

✅ Correct​

const total = 0;

Additional examples​

// ❌ Reassignment with let
let runningTotal = 0;
for (const value of values) {
    runningTotal += value;
}

// ✅ Declarative accumulation
const runningTotal = values.reduce((sum, value) => sum + value, 0);

ESLint flat config example​

import immutable from "eslint-plugin-immutable-2";

export default [
    {
        files: ["**/*.{js,mjs,cjs,ts,mts,cts,tsx}"],
        plugins: { immutable },
        rules: {
            "immutable/no-let": "error",
        },
    },
];

When not to use it​

let is still practical in tightly scoped algorithms where reassignment improves clarity (for example, pointer-based parsing loops or numeric simulations). If those hotspots are rare, keep no-let enabled globally and add selective per-file overrides.

Rule catalog ID: R905

Further reading​