Javascript: Why 'use strict';

Every concept in Javascript has too much information to keep in mind. Even "use strict;" as well. I can write lengthy article on it but I am a kind of guy usually ignore lengthy theoretical articles. So, this time I want to write article in different way but like one stop for "use strict;". No further words. Let's start:

What is "use strict;"?

To indicate code should run in strict mode - Thatz it. Yes 😊

How to use?

Everywhere in article writing it as "use strict;". Just place the same to the beginning of function or script.

When it is introduced in Javascript?

Do we really required?🧐 anyhow, In ES 5.

Please don't ask me, who introduced it. 😛

why do I use?

It makes to write secure Javascript. Short answers are not accepted for questions starting with Why😳.

Alright, Let's go deep but no theory.

S.No	Action	Example	Strict Mode	Non - Strict Mode	Fix
1	Using variable without declaring it	"use strict"; a = 3.14; console.log(a); view raw undeclaredVar.js hosted with ❤ by GitHub	Uncaught ReferenceError: a is not defined	3.14	"use strict" let a = 3.14; console.log(a); view raw undeclaredVarFix.js hosted with ❤ by GitHub
2	Deleting a variable	'use strict'; let a = 10; delete a; view raw deleteVariable.js hosted with ❤ by GitHub	Uncaught SyntaxError: Delete of an unqualified identifier in strict mode.	false	It can't be deleted. Variables created without var, let & const are deleted using delete.
3	Duplicate function params	'use strict'; function a(x, x) { console.log(x,x); } a(10, 10); view raw duplicateParam.js hosted with ❤ by GitHub	Uncaught SyntaxError: Duplicate parameter name not allowed in this context	10 10	'use strict'; function a(x, y) { console.log(x,y); } a(10, 10); view raw duplicateParamFix.js hosted with ❤ by GitHub
4	Octal numeric literals	'use strict'; let a = 010; console.log(a); view raw octalvalue.js hosted with ❤ by GitHub	Uncaught SyntaxError: Octal literals are not allowed in strict mode.	8	'use strict'; let a = 0o10; console.log(a); // OR 'use strict'; let a = parseInt('010', 8); console.log(a); view raw octalValueFix.js hosted with ❤ by GitHub
5	Using variable name as eval or arguments	'use strict'; let eval = 10; let arguments = 20; console.log(eval + ' ' + arguments); view raw evalArgs.js hosted with ❤ by GitHub	Uncaught SyntaxError: Unexpected eval or arguments in strict mode	10 20	'use strict'; let a = 10; let b = 20; console.log(a + ' ' + b); view raw evalArgsFix.js hosted with ❤ by GitHub
6	`this` inside function - refers to the object that called the function.	"use strict"; function testFn() { console.log(this); } testFn(); view raw thisInsideFn.js hosted with ❤ by GitHub	undefined	Returns window object: Window {0: Window, 1: Window, 2: global,......	Avoid using this inside functions to not to expose window object.
7	Creating variable with `eval`	'use strict'; eval('var a =10'); console.log(a); view raw varinEval.js hosted with ❤ by GitHub	Uncaught ReferenceError: a is not defined	10	Avoid using eval.
8	Using `with` statement	'use strict'; function test(a) { with (a) { console.log(a) } } test(20); view raw with.js hosted with ❤ by GitHub	Uncaught SyntaxError: Strict mode code may not include a with statement	20	Avoid using it
9	Deleting undeletable property	'use strict'; delete Object.prototype; view raw deleteProtoType.js hosted with ❤ by GitHub	Uncaught TypeError: Cannot delete property 'prototype' of function Object() { [native code] } at :2:1	false	Avoid it
10	Using Octal Escape characters	'use strict'; let a = '\010'; console.log(a); view raw useOctalEscape.js hosted with ❤ by GitHub	Uncaught SyntaxError: Octal escape sequences are not allowed in strict mode.	Returns nothing	Avoid it
11	Writing to read only property	"use strict"; const obj = {}; Object.defineProperty(obj, "a", {value:10, writable:false}); obj.a = 20; console.log(obj.a); view raw readonly.js hosted with ❤ by GitHub	Uncaught TypeError: Cannot assign to read only property 'a' of object '#'	10	"use strict"; const obj = {}; Object.defineProperty(obj, "a", {value:10, writable:true}); obj.a = 20; console.log(obj.a); view raw makeWritable.js hosted with ❤ by GitHub
12	Writing to get only property	'use strict'; const obj = { word: "first", get test() { return this.word.toUpperCase(); } }; obj.test = 'test'; console.log(obj.test); view raw getOnly.js hosted with ❤ by GitHub	Uncaught TypeError: Cannot set property test of # which has only a getter	FIRST	'use strict'; const obj = { word: "first", get test() { return this.word.toUpperCase(); }, set test(x) { this.word = x; } }; obj.test = 'test'; console.log(obj.test); view raw getOnlyFix.js hosted with ❤ by GitHub
13	Using variable as reserved keywords	'use strict'; let public = 10; console.log(public); view raw reservedKeyword.js hosted with ❤ by GitHub	Uncaught SyntaxError: Unexpected strict mode reserved word	10	Avoid using reserved keywords as variables

Oops, it took whole day to write. Anyhow, please do comment if I miss any or in case of improvements.

Thanks.

---

💎 Love to see your response

1. Like - You reached here means. I think, I deserve a like.
2. Comment - We can learn together.
3. Share - Makes others also find this resource useful.
4. Subscribe / Follow - to stay up to date with my daily articles.
5. Encourage me - You can buy me a Coffee

---

Let's discuss further.

1. Just DM @urstrulyvishwak

2. Or mention
   @urstrulyvishwak

For further updates:

Follow @urstrulyvishwak