this in global scope in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does this refer to in the global scope in JavaScript?

In the global scope, this refers to the global object. In browsers, this is usually the window object.

intermediate

How does this behave differently in strict mode in the global scope?

In strict mode, this in the global scope is undefined instead of the global object.

intermediate

What is the global object in Node.js when using this in the global scope?

In Node.js, the global object is global, but this in the top-level global scope of a module is not global; it is an empty object {}.

beginner

Code snippet:

console.log(this);

What will this print in a browser's global scope?

It will print the window object, which is the global object in browsers.

beginner

Why is understanding this in the global scope important?

Because it helps you know what object your code is referring to, avoiding bugs and unexpected behavior especially when switching between strict mode and non-strict mode or different environments.

In a browser, what does this refer to in the global scope (non-strict mode)?

A document object

B undefined

C null

D window object

this in the global scope refers to the global object, which is window in browsers.

What is the value of this in the global scope when running JavaScript in strict mode?

A undefined

B window object

C global object

D null

In strict mode, this in the global scope is undefined.

In Node.js modules, what does this refer to at the top-level global scope?

A an empty object

B module.exports

C global object

D undefined

In Node.js modules, this at the top-level is an empty object, not the global object.

Which of the following is true about this in the global scope?

A <code>this</code> always refers to the global object

B <code>this</code> is always undefined

C <code>this</code> can be undefined in strict mode

D <code>this</code> refers to the current function

In strict mode, this in the global scope is undefined; otherwise, it refers to the global object.

What will console.log(this === window); output in a browser's global scope (non-strict mode)?

A false

B true

C undefined

D TypeError

In the browser global scope, this is the window object, so the comparison is true.

Explain what this refers to in the global scope in JavaScript and how strict mode changes it.

Think about what object <code>this</code> points to when you run code outside any function.

Describe how this behaves differently in the global scope between browser and Node.js environments.

Consider the global objects and module system in both environments.

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