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Elevating JavaScript: The Power of 'Use Strict' Mode

Anchal Rastogi
~ 6 min read | Published on Apr 12, 2024


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In the vast and ever-evolving landscape of web development, JavaScript stands as a towering presence, powering the dynamism and interactivity of modern websites and applications. With its flexibility comes great responsibility, prompting the introduction of 'use strict'; a directive that helps developers write safer and more predictable code. This article delves deep into strict mode in JavaScript, offering seasoned web developers insights into its advantages, how to implement it, and why it could become an indispensable part of your coding toolkit.

Understanding 'Use Strict' in JavaScript

Introduced in ECMAScript 5, strict mode provides a way to opt into a restricted variant of JavaScript. By enforcing stricter parsing and error handling on your scripts, it helps catch common coding bloopers, discouraging the use of potentially problematic syntax and features. When strict mode is in effect, JavaScript will throw more exceptions, preventing silent failures and making it easier to debug and maintain code.

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Why Embrace Strict Mode?

Strict mode was introduced to accomplish several key objectives:

  • Eliminate JavaScript silent errors by changing them to throw errors, improving code debugging.
  • Fix mistakes that make it difficult for JavaScript engines to perform optimizations. Code running in strict mode can sometimes run faster than identical code that's not strict mode.
  • Prohibit syntax likely to be defined in future versions of ECMAScript, ensuring that your code is forward-compatible.

Enabling Strict Mode

Enabling strict mode is straightforward. For an entire script, add "use strict"; at the top of a JavaScript file or function. This tells the JavaScript engine to execute the code in strict mode:

"use strict";
function canYouSpotTheError() {
  undeclaredVariable = 'This will throw an error in strict mode';

Scope of Strict Mode

Strict mode can be applied to entire scripts or individual functions, but not to block statements enclosed in {} braces. If you want to apply it to a specific function, include "use strict"; at the beginning of the function body:

function myStrictFunction() {
  "use strict";
  // Function-level strict mode syntax
  // This code runs in strict mode

Key Behaviors of Strict Mode

Variables Must Be Declared

Strict mode requires that all variables be declared before they are used. Attempting to use an undeclared variable will result in a ReferenceError:

"use strict";
x = 3.14; // ReferenceError: x is not defined

Assignments to Non-Writable Properties Are Not Allowed

In strict mode, an attempt to assign a value to a non-writable property results in a TypeError:

"use strict";
var obj = Object.freeze({name: "John"}); = "Doe"; // TypeError: Cannot assign to read only property 'name'

Creating Properties on Primitive Values Throws an Error

Attempting to add properties to a primitive value (e.g., number, string, boolean) will throw an error:

"use strict";
false.true = "";         // TypeError
(14).sailing = "home";   // TypeError
"with".you = "Far away"; // TypeError

The this Value Is Not Coerced to the Global Object

In non-strict mode, this refers to the global object when a function is called without an explicit this value. In strict mode, this remains undefined, making it easier to spot unintended behavior:

"use strict";
function whatIsThis() {
  return this;
console.log(whatIsThis()); // undefined

Strict Mode and Your Development Workflow

Incorporating strict mode into your development practice can significantly improve the quality of your code. It encourages the adoption of good coding habits, reduces the chance of errors, and can even improve performance. However, transitioning existing projects to strict mode should be approached with caution, as it can reveal previously unnoticed errors.

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Integrating with Zipy's Monitoring Tools

While strict mode is a powerful tool for improving code quality, it's just one part of maintaining a healthy codebase. Zipy's toolset offers comprehensive monitoring solutions, including error tracking and session replay capabilities. By combining strict mode with Zipy's monitoring tools, developers can achieve greater insight into their applications' behavior and performance, leading to faster debugging and a better user experience. Discover how Zipy can elevate your web development process at Discover Zipy's Full Potential.


The 'use strict' directive in JavaScript represents more than just a set of constraints; it's a commitment to cleaner code, fewer errors, and a more efficient development process. By understanding and implementing strict mode in your projects, you're not only future-proofing your code but also embracing best practices that stand to benefit developers, users, and the JavaScript community at large. Coupled with advanced tools like Zipy for monitoring and debugging, strict mode enables developers to craft robust, high-quality web applications that stand the test of time.

Through this comprehensive exploration of strict mode in JavaScript, developers are equipped with the knowledge to harness its full potential, enhancing both their code and their coding practices.

Read more resources Javascript concepts

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