What is strict mode in JavaScript? What are the differences when it is enabled and when it is not? What restrictions does it impose?

Strict mode in JavaScript is a special mode that introduces stricter rules for syntax and code behavior, helping to avoid common errors and improve security.

Differences of strict mode:

• Enabled by the directive 'use strict'; at the beginning of a script or function.
• In strict mode, some unsafe or deprecated constructs are disallowed.

Main restrictions and changes in strict mode:

• Prohibition of using undeclared variables (e.g., x = 10; without var/let/const will cause an error).
• Cannot delete variables, functions, or arguments (delete x; will cause an error).
• Prohibition of duplicate parameter names in functions.
• The value of this in functions called without a context will be undefined instead of the global object.
• Prohibition of using reserved words as variable names (e.g., implements, interface).
• Prohibition of assigning values to read-only or non-existent properties.

Example:

'use strict';

function test() {
  x = 10; // Error: x is not declared
}

test();

Without strict mode, such code would create a global variable x, which can lead to hard-to-find bugs.

Thus, strict mode helps write more reliable and predictable code.