Javascript Strict Mode

Updated April 11, 2023

Introduction to Javascript Strict Mode

Strict Mode is a new feature being introduced in JavaScript with the advent of ES5 changes. It changes the whole schema of ordinary code in JavaScript and makes it more of a “strict” operating category. The Strict context makes the JavaScript code more able to deny certain actions and throw more exceptions. This mode can be implemented within the code by writing “use strict” which invokes the browser to operate in the strict mode which is a safer and better mode to work in. Some browsers which are not supporting the strict mode will behave differently when strict mode is used inside them hence a prior feasibility testing of the browser should be also done before working in a strict mode. Strict mode code and no-script mode code can co-occur at the same time in the code hence the scripts can opt to strict mode slowly and incrementally.

Syntax

We can state any JavaScript code to be used in Strict Mode by just using a sentence “use strict” in supported browsers as follows:

'use strict';
var x = 4; //Valid in strict mode
y = 1; //invalid in strict mode

Benefits of Using Strict Mode

Strict mode offers multiple changes to a normal JavaScript code changing its semantics and making it more controlled via error throwing logic. Some of the main benefits of its introduction in JavaScript are mentioned below:

• The Strict mode help eliminate some of the silent errors in JavaScript by raising a throw error statement in such cases.

• The Strict Mode throws, or prevent errors, whenever relatively “unsafe” actions are being taken within the JavaScript Code (i.e. accessing a global object or initializing a variable without defining it).
• The Strict mode basically fixes faults in the strict context that makes it easier for the JavaScript engine to carry out optimizations. Hence, strict mode JavaScript code can erratically be running quicker than the same JavaScript code that is running in non-strict mode.
• Some of the syntax and semantics are likely to be added in JavaScript with future versions of ECMAScript hence it would be easier if the foundation of those changes is prohibited by Strict Mode.
• The Strict Mode can be invoked in scripts, functions, and modules as well.

Strict Mode in Scripts:

'use strict';
var x = 'Strict Mode in Scripts!';

Strict Mode in Functions:

const Strict = () => {
'use strict';
return "Strict Mode in Functions!"
}

Strict mode in Modules:

function method( ) {
// being a module it is by default strict in nature
}
export default method;

• The Strict mode basically helps writing strict and secure JavaScript code.
• The Strict Mode has disabled features that were confusing or poorly thought out.

Examples of Javascript Strict Mode

Here are the following examples mention below

Example #1 – Initializing an undefined variable

Code:

"use strict"
var v = 1 // error raised

Output:

Example #2 – Function level strict mode in action

Code:

"use strict";
myFunc();
function myFunc() {
y = 1;   // error raised
}

Output:

Example #3 – Declaring strict within a function

Code:

x = 1;    // not raise error
myFunction();
function myFunction() {
"use strict";
y = 1;  // error raised
}

Output:

Example #4 – Initializing undefined object

Code:

"use strict";
x = {y:1, z:2};   // error raised

Output:

Example #5 – Deleting a variable

Code:

"use strict";
var x = 1;
delete x;     // error raised

Output:

Example #6 – Deleting a function

Code:

"use strict";
function x(y, z) {};
delete x;        // error raised

Output:

Example #7 – Duplicating a parameter

Code:

"use strict";
function x(y, y) {};   // error raised

Output:

Example #8 – Using octal number literals

Code:

"use strict";
var v = 011;   // error raised

Output:

Example #9 – Using octal escape characters

Code:

"use strict";
var x = "\011";   // error raised

Output:

Example #10 – Modifying the read-only property

Code:

"use strict";
var obj = {};
Object.defineProperty(obj, "y", {value:1, writable:false});
obj.y = 10;   // error raise

Output:

 

Example #11 – Modifying the get-only property

Code:

"use strict";
var obj = {get y() {return 0} };
obj.y = 1;   // error raised

Output:

Example #12 – Deleting an object prototype

Code:

"use strict";
delete Object.prototype;   // error raised

Output:

Example #13 – Initializing variable as ‘eval’

Code:

"use strict";
var eval = 1;   // error raised

Output:

 

Example #14 – Initializing variable as ‘arguments’

Code:

"use strict";
var arguments = 1;   // error raised

Output:

Example #15 – Using ‘with’ statement’

Code:

"use strict";
with (Math){y = sin(2)};   // error raised

Output:

Example #16 – Using ‘eval’ to create variables within the scope

Code:

"use strict";
eval ("var y = 1");
alert (y);      // error raised

Output:

Example #17 – Use of ‘this’ keyword in functions of strict mode

The ‘this’ keyword is used for referring the object that called some function. If any object is undefined in a function then in strict mode, the function will return ‘undefined’ for the object and functions working in non-strict mode will rather return the global value of the object (if defined globally).

Code:

"use strict";
function myFunc() {
alert(this);
}
myFunc();

Example #18 – Using reserved words of future JavaScript versions for defining a variable

Code:

"use strict";
var public = 1;   // error raised

Output:

Code:

"use strict";
var implements= 1;   // error raised

Output:

Code:

"use strict";
var interface= 1;   // error raised

Output:

Code:

"use strict";
var let= 1;   // error raised

Output:

Code:

"use strict";
var package= 1;   // error raised

Output:

Code:

"use strict";
var private= 1;   // error raised

Output:

Code:

"use strict";
var protected= 1;   // error raised

Output:

Code:

"use strict";
var static= 1;   // error raised

Output:

Code:

"use strict";
var yield= 1;   // error raised

Output:

Conclusion

Strict Mode is an exciting feature introduced with ECMAScript 5.0 which has enhanced the semantics and also added extra secureness to the JavaScript coding platform. It has decreased the fault tolerance in JavaScript and also makes it easy for the developers to write code in JavaScript without worrying about some of the common errors which are explicitly thrown by it.

Recommended Articles

This is a guide to Javascript Strict Mode. Here we discuss the Examples of Javascript Strict Mode along with the benefits. You may also have a look at the following articles to learn more –

1. JavaScript FileReader
2. JavaScript Array Concat
3. Nested Loop in JavaScript
4. JavaScript String to int