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Rslint

    no-undefined

    Added in v0.8.1

    Configuration

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

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

    Rule Details

    Disallow the use of undefined as an identifier.

    The undefined variable in JavaScript is actually a property of the global object. As such, in ECMAScript 3 it was possible to overwrite the value of undefined. While ECMAScript 5 disallows overwriting undefined, it's still possible to shadow undefined, such as:

    function doSomething(data) {
  const undefined = "hi";

  // doesn't do what you think it does
  if (data === undefined) {
    // ...
  }
}

    Because undefined can be overwritten or shadowed, reading undefined can give an unexpected value. (This is not the case for null, which is a keyword that always produces the same value.) To guard against this, you can avoid all uses of undefined, which is what some style guides recommend and what this rule enforces. Those style guides then also recommend:

    • Variables that should be undefined are simply left uninitialized. (All uninitialized variables automatically get the value of undefined in JavaScript.)
    • Checking if a value is undefined should be done with typeof.
    • Using the void operator to generate the value of undefined if necessary.

    Examples of incorrect code for this rule:

    const foo = undefined;

const undefined = "foo";

if (foo === undefined) {
  // ...
}

function baz(undefined) {
  // ...
}

bar(undefined, "lorem");

    Examples of correct code for this rule:

    const foo = void 0;

const Undefined = "foo";

if (typeof foo === "undefined") {
  // ...
}

global.undefined = "foo";

bar(void 0, "lorem");

    Options

    This rule has no options.

    Differences from ESLint

    • A named class declaration whose name is undefined (class undefined {}) is reported once. ESLint reports the same position twice.
    • An assignment-destructuring pattern with a shorthand default whose name is undefined (e.g. ({ undefined = 1 } = target)) is reported once. ESLint reports the same position twice.

    Original Documentation