JavaScript's strict mode, introduced in ECMAScript 5, optimizes code for JS engine, eliminates bad-syntax code, and helps in writing secure JavaScript. Let's take a slightly deeper dive into this concept.

"Use strict" mode in JS applies certain restrictions in the entire script or in a particular function it is invoked in. Just add "use strict"; to the beginning of a script or a function.

Q. What changes does use strict make? The list is pretty exhaustive. But here are some important changes you'll notice. If you're in strict mode:

• Using a variable/object, without declaring it, will throw an error.

• Using duplicate parameters in any function or duplicate property names in any object will throw an error.

• Any assignment that silently fails (does not throw any error/no feedback to the user) in normal code (i.e assignment to a non-writable global or property, assignment to a getter-only property, assignment to a new property on a non-extensible object) will throw an error.

• Deleting a function or an undeletable property will throw an error.

• Using a number that begins with 0 will throw an error.

• Setting a property to a primitive value, will throw an error.

• There are a number of situations that could unintentionally cause this to be bound to the global object. In strict mode, instead of bounding this to global object, this gets bound to undefined.

• ECMAScript 5 added a list of reserved words that will be used in the future. If you use them as variables or arguments, the strict mode will throw an error.

• Using arguments.callee, arguments.caller, with keyword,eval() function, is forbidden.

• Et cetera, et cetera.

Refer to this MDN doc on the same for further information and clarity.

Happy coding :)

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