# svelte/prefer-const

Require const declarations for variables that are never reassigned after declared

🔧 The --fix option on the command line can automatically fix some of the problems reported by this rule.

# 📖 Rule Details

This rule reports the same as the base ESLint prefer-const rule, except that ignores Svelte reactive values such as $derived and $props as default. If this rule is active, make sure to disable the base prefer-const rule, as it will conflict with this rule.

<script>
  /* eslint svelte/prefer-const: "error" */

  // ✓ GOOD
  const { a, b } = $props();
  let c = $state('');
  let d = $derived(a * 2);
  let e = $derived.by(() => b * 2);

  // ✗ BAD
  let 'obj' is never reassigned. Use 'const' instead. (svelte/prefer-const)obj = { a, b };
  let 'g' is never reassigned. Use 'const' instead. (svelte/prefer-const)g = $state(0);
  let 'h' is never reassigned. Use 'const' instead. (svelte/prefer-const)h = $state({ count: 1 });
</script>

<input bind:value={c} />
<input bind:value={h.count} />

# 🔧 Options

{
  "svelte/prefer-const": [
    "error",
    {
      "destructuring": "any",
      "additionalProperties": false,
      "excludedRunes": ["$props", "$derived"]
    }
  ]
}

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.
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.
excludedRunes: An array of rune names that should be ignored. Even if a rune is declared with let, it will still be ignored.

# 📚 Further Reading

See ESLint prefer-const rule for more information about the base rule.

# 🚀 Version

This rule was introduced in eslint-plugin-svelte v3.0.0-next.6

# 🔍 Implementation

^{Taken with ❤️ from ESLint core}