# Why do we enforce use == over === in JavaScript?

You might think we're crazy at first, but let me explain everything in detail.

An ESLint rule was introduced three weeks ago to ensure engineers consistently use `==` instead of `===` when checking for `null` or `undefined`.

```javascript
// null check guidance
    'no-restricted-syntax': [
      'error',
      {
        selector:
          "BinaryExpression[operator='===']:matches([left.type='Literal'][left.value=null],[right.type='Literal'][right.value=null])",
        message:
          'Use == null. `foo === null` returns `false` if `foo` is `undefined`, which is generally not what you want.',
      },
      {
        selector:
          "BinaryExpression[operator='!==']:matches([left.type='Literal'][left.value=null],[right.type='Literal'][right.value=null])",
        message:
          'Use != null. `foo != null` is true if `foo` is neither null nor undefined, which is usually what you want.',
      },
      {
        selector:
          "BinaryExpression[operator='===']:matches([left.type='Identifier'][left.name='undefined'],[right.type='Identifier'][right.name='undefined'])",
        message:
          'Use == null. `foo === undefined` returns `false` if `foo` is `null`, which is generally not what you want.',
      },
      {
        selector:
          "BinaryExpression[operator='!==']:matches([left.type='Identifier'][left.name='undefined'],[right.type='Identifier'][right.name='undefined'])",
        message:
          'Use != null. `foo != null` is true if `foo` is neither null nor undefined, which is usually what you want.',
      },
    ],
```

The root cause lies in the fact that `null` is not equal to `undefined`. Most of the time, we simply want to verify that a variable is neither `null` nor `undefined`. However, using `===` for the check can lead to unexpected results, potentially causing more issues.

<div data-node-type="callout">
<div data-node-type="callout-emoji">💡</div>
<div data-node-type="callout-text">This rule applies only to expressions involving <code>null</code> or <code>undefined</code>; for all other conditions, we strongly recommend using <code>===</code> to prevent unexpected type conversions.</div>
</div>