Lightning Aura Components Developer Guide

Introducing Aura Components
Quick Start
Creating Components
Using Components
Communicating with Events
Communicating Across the DOM with Lightning Message Service
Creating Apps
Styling Apps
Developing Secure Code
Lightning Locker
JavaScript Strict Mode Enforcement
DOM Access Containment
Secure Wrappers
eval() Function is Limited by Lightning Locker
MIME Types Permitted
Access to Supported JavaScript API Framework Methods Only
What Does Lightning Locker Affect?
Lightning Locker Tools
Select the Locker API Version for an Org
Disable Lightning Locker for a Component
Don’t Mix Component API Versions
Lightning Locker Disabled for Unsupported Browsers
Lightning Web Security
Content Security Policy Overview
Using JavaScript
Working with Salesforce Data
Testing Components
Debugging
Performance
Reference
PDF

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JavaScript Strict Mode Enforcement

Lightning Locker implicitly enables JavaScript strict mode. You don’t need to specify "use strict" in your code. JavaScript strict mode makes code more secure, robust, and supportable.

When strict mode is enabled and unsafe actions are taken, JavaScript throws errors that are otherwise suppressed. Examples of unsafe actions include assigning values to non-writable properties and using a variable that hasn’t been declared. Reporting these actions can catch situations when a variable name has been mistyped.

A few common stumbling points when using strict mode are:

  • You must declare variables with the var or let keyword.
  • You must explicitly attach a variable to the window object to create a global variable that’s available across components or libraries. For more information, see Sharing JavaScript Code Across Components.
  • The libraries that your components use must also work in strict mode.

For more information about JavaScript strict mode, see the Mozilla Developer Network.