implicitGlobals

Prevents implicit global variable declarations in browser scripts.

✅ This rule is included in the browser logical preset.

In browser scripts (non-module code), top-level var declarations and function declarations create properties on the global window object. This can lead to unexpected behavior and naming conflicts with other scripts loaded in the same page.

This rule flags top-level var declarations and function declarations in non-module scripts. It encourages developers to either use modules (with import/export), use let or const declarations (which don’t create globals), or explicitly assign to window if global access is truly needed.

Examples

❌ Incorrect

var globalCounter = 0;

function incrementCounter() {
  globalCounter++;
}

✅ Correct

// Use let or const - they don't create window properties
let globalCounter = 0;

function incrementCounter() {
  globalCounter++;
}

// Or use a module with export
export const globalCounter = 0;

export function incrementCounter() {
  // ...
}

// Or explicitly assign to window if truly needed
window.globalCounter = 0;

window.incrementCounter = function () {
  window.globalCounter++;
};

Options

This rule is not configurable.

When Not To Use It

If you’re working with legacy non-module browser code where globals are intentional and there’s no risk of naming conflicts, you might disable this rule. For modern applications, prefer using modules and avoiding implicit globals.

Further Reading

• MDN: JavaScript modules
• MDN: var
• MDN: let
• MDN: const

Equivalents in Other Linters

• ESLint: no-implicit-globals