Can you provide a brief explanation of strict mode in JavaScript and the purpose of the 'use strict' directive? Additionally, can you clarify if this directive introduces new features and whether or not all browsers support it? If so, what are these features and how do they impact JavaScript code?

JavaScript has historically added new features to the language without backwards compatibility issues. However, this approach resulted in mistakes and imperfect decisions from the creators remaining in the language forever, which was addressed by ECMAScript 5 (ES5) in 2009. Though ES5 added new features and modified existing ones, such changes were not applied by default to ensure outdated code continued to function properly. Instead, developers needed to use the "use strict" directive to activate these changes.

When placed at the beginning of a script, the entire script runs in "modern" mode, ensuring strict mode applies throughout. Alternatively, it can be added to most types of functions to activate strict mode only within that function. However, "use strict" must be the first executable line of code in the script to properly activate strict mode. Once enabled, there is no way to cancel it, and it is important to note that the browser console may not have strict mode enabled by default.

While starting each script with "use strict" is advisable for now, modern JavaScript structures like classes and modules automatically enable strict mode and do not require the directive.

Strict mode is a feature in JavaScript that was introduced in ECMAScript 5. It provides a way to opt into a stricter set of rules and best practices for writing JavaScript code. By enabling strict mode, you can avoid common mistakes and prevent certain types of errors.

The 'use strict' directive is used to enable strict mode in JavaScript. Placing this directive at the beginning of a script or function enables strict mode for that specific context. When enabled, strict mode enforces a different set of rules for several aspects of JavaScript, such as variable declarations, assignments, function parameters, and more.

One of the main purposes of strict mode is to make JavaScript more predictable and less error-prone. It catches common coding mistakes by turning them into errors that would otherwise be silent failures or allowed behavior in non-strict mode. This helps improve code quality and maintainability.

Strict mode also introduces a few new features and changes some existing behaviors. For example, it disallows the use of variables without declaring them first (e.g., assigning a value to an undeclared variable will throw an error). It also prevents the accidental global scope pollution caused by implicit global variable declarations.

Regarding browser support, almost all modern browsers, as well as Node.js, support strict mode. However, older browsers that do not support ECMAScript 5 may not fully support strict mode. In such cases, using the 'use strict' directive will not cause any syntax errors, but the new features and stricter behavior may not apply. To ensure compatibility, it's recommended to use a tool like Babel or a transpiler that can convert your code to an older version of JavaScript that works across a wide range of browsers.