Why this in global scope in Javascript? - Purpose & Use Cases

The Big Idea

What if the simple word <code>this</code> could secretly cause your code to break in mysterious ways?

The Scenario

Imagine you want to access a global value or function inside your JavaScript code, but you try to do it by guessing where this points to. You write code assuming this always means the global object, but sometimes it doesn't, causing confusion and bugs.

The Problem

Manually tracking what this refers to in different parts of your code is slow and error-prone. In the global scope, this can behave differently depending on strict mode or environment, leading to unexpected results and wasted debugging time.

The Solution

Understanding how this works in the global scope helps you predict its value reliably. You learn that in the global scope, this usually points to the global object (like window in browsers), unless strict mode changes that. This clarity prevents bugs and makes your code easier to read and maintain.

Before vs After

✗ Before

console.log(this);
// Unclear if this is global object or undefined in strict mode

✓ After

console.log(this === window);
// true in browser global scope, clear meaning

What It Enables

Knowing this in the global scope lets you write predictable, bug-free code that interacts correctly with global variables and functions.

Real Life Example

When you write a script that runs in a browser and want to access or set global variables, understanding this in the global scope helps you avoid mistakes like accidentally creating local variables or getting undefined instead of the global object.

Key Takeaways

this in global scope usually points to the global object like window.

Strict mode can change this to undefined, causing confusion.

Knowing this behavior prevents bugs and makes your code clearer.

Practice

1. In JavaScript, what does this refer to when used in the global scope (outside any function) in a browser environment?

easy

A. The document object

B. The current function

C. The global object, which is window in browsers

D. Undefined

Solution

1. Step 1: Understand global scope context

   In the global scope, this refers to the global object in JavaScript.

2. Step 2: Identify the global object in browsers

   In browsers, the global object is window, so this points to window.

3. Final Answer:

   The global object, which is window in browsers -> Option C

4. Quick Check:

   this in global scope = window [OK]

Hint: Remember: global this is window in browsers [OK]

Common Mistakes:

• Thinking this is undefined in global scope by default
• Confusing this with the current function
• Assuming this points to document

2. Which of the following is the correct way to check if this in the global scope refers to the window object?

easy

A. console.log(this === document);

B. console.log(this === window);

C. console.log(this === global);

D. console.log(this === null);

Solution

1. Step 1: Recall global this in browsers

   In the browser global scope, this points to the window object.

2. Step 2: Compare this with window

   Using this === window returns true if this is the global object.

3. Final Answer:

   console.log(this === window); -> Option B

4. Quick Check:

   this === window is true in global scope [OK]

Hint: Check this === window to confirm global this [OK]

Common Mistakes:

• Using document instead of window
• Using global which is Node.js specific
• Comparing this to null

3. What will be the output of the following code in a browser's global scope?

console.log(this === window);
'use strict';
console.log(this);

medium

A. true and then global object

B. true and then undefined

C. false and then window object

D. true and then window object

Solution

1. Step 1: Evaluate first console.log before strict mode

   Before 'use strict', this in global scope equals window, so this === window is true.

2. Step 2: Evaluate second console.log after strict mode

   Strict mode affects this inside functions, but in global scope this still points to window, so it logs the window object.

3. Final Answer:

   true and then window object -> Option D

4. Quick Check:

   Global this = window even with strict mode [OK]

Hint: Strict mode doesn't change global this in browsers [OK]

Common Mistakes:

• Assuming this becomes undefined in global scope with strict mode
• Confusing strict mode effects inside functions vs global scope
• Thinking this equals global in browsers

4. Consider this code snippet:

'use strict';
function checkThis() {
  console.log(this);
}
checkThis();

What is the problem if you want to log the global object using this inside checkThis?

medium

A. this is undefined because of strict mode inside the function

B. this points to the global object as expected

C. this points to the function itself

D. Syntax error due to strict mode

Solution

1. Step 1: Understand strict mode effect inside functions

   In strict mode, this inside a regular function called without an object is undefined.

2. Step 2: Check the function call context

   The function checkThis is called directly, so this is not bound to the global object.

3. Final Answer:

   this is undefined because of strict mode inside the function -> Option A

4. Quick Check:

   Strict mode makes this undefined in functions called globally [OK]

Hint: Strict mode sets this to undefined inside functions [OK]

Common Mistakes:

• Assuming this always points to global object
• Thinking strict mode causes syntax errors here
• Confusing function this with global this

5. You want to create a global variable myGlobal and access it via this.myGlobal in the global scope under strict mode. Which code snippet correctly achieves this in a browser environment?

hard

A. window.myGlobal = 10; console.log(this.myGlobal);

B. let myGlobal = 10; console.log(this.myGlobal);

C. const myGlobal = 10; console.log(this.myGlobal);

D. var myGlobal = 10; console.log(this.myGlobal);

Solution

1. Step 1: Understand variable declaration and global object

   In strict mode, variables declared with var, let, or const do not become properties of window.

2. Step 2: Check how to explicitly add property to global object

   Assigning window.myGlobal = 10 explicitly adds myGlobal as a property of the global object.

3. Final Answer:

   window.myGlobal = 10; console.log(this.myGlobal); -> Option A

4. Quick Check:

   Only window.myGlobal ensures this.myGlobal exists globally [OK]

Hint: Use window.myGlobal to add global property reliably [OK]

Common Mistakes:

• Using let or const expecting global this property
• Assuming var always adds property to this in all contexts
• Not understanding difference between variable and global object property