#no-implicit-coercion

#Configuration

import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'no-implicit-coercion': 'error',
    },
  },
]);

Disallow shorthand type conversions in favor of explicit Boolean() / Number() / String() calls.

#Rule Details

In JavaScript, type conversions are often performed using shorthand syntax. While these idioms work, they obscure intent; the rule flags them so they can be replaced with explicit conversions.

Patterns flagged:

• !!foo instead of Boolean(foo)
• ~foo.indexOf(bar) (or lastIndexOf) instead of foo.indexOf(bar) !== -1
• +foo or -(-foo) instead of Number(foo)
• 1 * foo or foo * 1 instead of Number(foo)
• foo - 0 instead of Number(foo)
• "" + foo / foo + "" (including ``) instead of String(foo)
• foo += "" instead of foo = String(foo)
• Template shorthand `${foo}` instead of String(foo) — only when the disallowTemplateShorthand option is enabled.

#Options

{
  "no-implicit-coercion": [
    "error",
    {
      "boolean": true,
      "number": true,
      "string": true,
      "disallowTemplateShorthand": false,
      "allow": []
    }
  ]
}

• boolean (default true) — disallow boolean shorthand conversions (!!, ~).
• number (default true) — disallow numeric shorthand conversions (+, - -, - 0, * 1).
• string (default true) — disallow string shorthand conversions ("" +, += "").
• disallowTemplateShorthand (default false) — also disallow `${foo}` as a coercion.
• allow — operators to exempt from the above. Allowed values: "~", "!!", "+", "- -", "-", "*".

#Fix vs suggestion

Only !!foo → Boolean(foo) applies as an autofix (and only when Boolean is not shadowed in scope). The other rewrites are offered as suggestions because they can change runtime behavior — Number(1n) throws, foo.indexOf(x) !== -1 differs from ~foo.indexOf(x) on non-array targets, etc.

#Examples

Incorrect:

const b = !!foo;
const b1 = ~foo.indexOf('.');
const n = +foo;
const n2 = foo - 0;
const n3 = 1 * foo;
const s = '' + foo;
foo += '';

Correct:

const b = Boolean(foo);
const b1 = foo.indexOf('.') !== -1;
const n = Number(foo);
const s = String(foo);

#Original Documentation

https://eslint.org/docs/latest/rules/no-implicit-coercion