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Strict Mode - JavaScript Interview Questions

JavaScript Fundamentals: Strict Mode

What is Strict Mode?​

View Answer:
Interview Response: Strict Mode is a mode in JavaScript that enforces stricter parsing and error handling rules. It was introduced in ECMAScript 5 and is designed to make code more secure and easier to optimize.

Code Example:

"use strict";

// Example 1: Variable without declaration (throws an error in strict mode)
x = 10;  // Throws a ReferenceError: x is not defined

// Example 2: Duplicate parameter names (throws an error in strict mode)
function multiply(a, b, a) {  // Throws a SyntaxError: Duplicate parameter name not allowed in this context
  return a * b;
}

// Example 3: Octal literals (throws an error in strict mode)
var num = 0123;  // Throws a SyntaxError: Octal literals are not allowed in strict mode

// Example 4: 'this' in non-method functions (has different behavior in strict mode)
function myFunction() {
  console.log(this);  // In strict mode, 'this' is undefined
}

myFunction();

When was ECMAScript 5 (ES5) released for use?​

View Answer:
Interview Response: ECMAScript 5 (ES5) was released in December 2009 as the fifth edition of the ECMAScript language specification, which defines the standard for JavaScript.

How do you explicitly enable ES5 features and modifications?​

View Answer:
Interview Response: ES5 features can be enabled by setting the "use strict" directive at the beginning of a JavaScript file, which enforces strict mode and activates ES5 features. You need to enable them with a special directive explicitly: "use strict".

Code Example:

"use strict";

// this code works the modern way
...

How you should implement strict mode in JavaScript?​

View Answer:
Interview Response: Strict mode is a feature in JavaScript that enables you to write safer and more efficient code. To implement strict mode in JavaScript, add the "use strict" directive at the beginning of a script or function.

Code Example:

Proper Usage:

  1. Strict mode in the whole script:
"use strict";
var x = 3.14; // This is okay
x = 3.14; // This is not okay without var, let or const

In this example, the "use strict"; declaration is at the top of the script, enabling strict mode for the entire script. When strict mode is enabled, JavaScript doesn't allow variables to be used unless they're declared with var, let, or const.

  1. Strict mode in a function:
function strictFunc() {
  "use strict";
  var x = 3.14; // This is okay
  y = 3.14; // This is not okay without var, let or const
}

In this example, strict mode is enabled only within strictFunc. The "use strict"; statement is at the top of the function, so strict mode applies to the entire function scope. Any variables not declared with var, let, or const will result in an error.

Incorrect Usage:

  1. Not placing the "use strict"; directive at the top:
var x = 3.14;
"use strict";

In this example, the "use strict"; statement is not at the top of the script, so strict mode isn't activated. The "use strict"; statement must be at the top of the script or function for strict mode to be enabled.

  1. Using strict mode with non-compliant syntax:
"use strict";
x = 0177; // Octal literals are not allowed in strict mode

In this example, an octal literal (a number prefixed with a zero) is used, which is not allowed in strict mode. This will throw an error.

Can you cancel strict mode later in your code?​

View Answer:
Interview Response: No, there is no directive like "no use strict" that reverts the engine to the old behavior. Once we enter strict mode, there is no going back.

Does the browser developer console implement strict mode by default?​

View Answer:
Interview Response: No, the browser developer console does not implement strict mode by default. Strict mode must be enabled explicitly by using the "use strict" directive. We must place it at the first console line for it to work.

How do you implement strict mode in the browser console?​

View Answer:
Interview Response: We must place it at the first console line for it to work, then add the rest of our code. You can also use a wrapper if you are using an older browser.

Technical Response: First, you can try to press Shift+Enter to input multiple lines and put “use-strict” on top. In Older browsers, you have to put it in a wrapper.

Code Example: works in most browsers, namely Firefox and Chrome.

'use strict';
// <Shift+Enter for a newline>

//  ...your code

// <Enter to run>

Code Example: In Older browsers, you have to put it in a wrapper.

(function () {
  'use strict';

  // ...your code here...
})();

Should we implement ‘use strict’ in modern applications?​

View Answer:
Interview Response: Yes, it's generally recommended to use "use strict" in modern applications as it helps catch errors and enforces stricter syntax and behavior in JavaScript.

Technical Response: Yes, it remains recommended to use strict mode in all modern applications. Although it may not be essential in some cases, we should provide the strict-mode functionality.

Are there any language structures that implement strict mode by default?​

View Answer:
Interview Response: Yes, ECMAScript 6 (ES6) modules and JavaScript classes are in strict mode by default. No need to explicitly write "use strict"; in these structures.

What are some benefits of using Strict Mode?​

View Answer:
Interview Response: Strict Mode in JavaScript enforces stricter rules for parsing and error handling, preventing common coding mistakes and making it easier to optimize code for better performance.

What actions are disallowed in strict mode in JavaScript?​

View Answer:
Interview Response: The use of undeclared variables, assigning values to non-writable properties, deleting non-configurable properties, and using duplicate parameter names in function declarations are disallowed in strict mode in JavaScript.