What execution context is in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an execution context in JavaScript?

An execution context is the environment where JavaScript code runs. It holds information like variables, functions, and the value of 'this'.

beginner

Name the three main types of execution contexts in JavaScript.

Global execution context, Function execution context, and Eval execution context.

intermediate

What does the 'this' keyword represent inside an execution context?

'this' refers to the object that is currently executing the code in that context.

intermediate

What happens when a function is called in terms of execution context?

A new function execution context is created and pushed onto the call stack to run the function's code.

intermediate

How does the JavaScript engine manage multiple execution contexts?

It uses a call stack to keep track of execution contexts, running the top one and removing it when done.

What is the first execution context created when JavaScript code runs?

A Eval execution context

B Function execution context

C Block execution context

D Global execution context

The global execution context is created first and represents the outermost environment.

When a function is called, what happens to the execution context?

A The global context is replaced

B The function runs without any execution context

C A new execution context is created and added to the call stack

D The previous context is deleted

Each function call creates a new execution context pushed onto the call stack.

Which of these is NOT part of an execution context?

A Call stack

B Lexical environment

C 'this' binding

D Variable environment

The call stack manages execution contexts but is not part of a single execution context.

What does the 'this' keyword refer to inside a function execution context?

A The object that called the function

B The global object always

C The function itself

D Undefined

'this' refers to the object that called the function, depending on how the function is called.

What is the role of the call stack in execution contexts?

A To store variables

B To keep track of execution contexts in order

C To execute code asynchronously

D To manage memory allocation

The call stack tracks which execution context is currently running and manages their order.

Explain what an execution context is and why it is important in JavaScript.

Think about the environment where your code runs.

Describe how the call stack and execution contexts work together when functions are called.

Imagine stacking plates when you call functions.

Practice

1. What is an execution context in JavaScript?

easy

A. The environment where JavaScript code runs and variables/functions are accessible

B. A special type of variable that stores data temporarily

C. A function that executes automatically when the page loads

D. A tool used to debug JavaScript code in the browser

Solution

1. Step 1: Understand the meaning of execution context

   Execution context is the environment where JavaScript code runs, controlling variable and function access.

2. Step 2: Compare options with the definition

   Only The environment where JavaScript code runs and variables/functions are accessible correctly describes this environment; others describe unrelated concepts.

3. Final Answer:

   The environment where JavaScript code runs and variables/functions are accessible -> Option A

4. Quick Check:

   Execution context = environment for code execution [OK]

Hint: Execution context = where code runs and variables live [OK]

Common Mistakes:

• Confusing execution context with variables
• Thinking it's a debugging tool
• Mixing it up with functions

2. Which of the following is a valid type of execution context in JavaScript?

easy

A. Global, Function, and Block execution contexts

B. Local, Remote, and Server execution contexts

C. Compile-time, Run-time, and Debug execution contexts

D. Synchronous, Asynchronous, and Parallel execution contexts

Solution

1. Step 1: Recall the types of execution contexts

   JavaScript has three main execution contexts: Global, Function, and Block.

2. Step 2: Match options with known types

   Only Global, Function, and Block execution contexts lists the correct types; others list unrelated or incorrect terms.

3. Final Answer:

   Global, Function, and Block execution contexts -> Option A

4. Quick Check:

   Execution context types = Global, Function, Block [OK]

Hint: Remember the three main contexts: Global, Function, Block [OK]

Common Mistakes:

• Confusing execution context types with networking terms
• Mixing compile/run-time concepts
• Thinking async types are execution contexts

3. Consider the code:

let x = 10;
function foo() {
  let y = 20;
  console.log(x + y);
}
foo();

Which execution context is active when console.log runs?

medium

A. Global execution context only

B. Function execution context of foo

C. Block execution context inside foo

D. No execution context is active

Solution

1. Step 1: Identify where console.log is called

   It is inside the function foo, so the function's execution context is active.

2. Step 2: Understand execution context hierarchy

   When foo runs, its function execution context is created and active during console.log.

3. Final Answer:

   Function execution context of foo -> Option B

4. Quick Check:

   Inside function = function execution context active [OK]

Hint: Code inside function runs in function execution context [OK]

Common Mistakes:

• Thinking only global context is active
• Confusing block context with function context
• Believing no context is active during execution

4. Find the error in this code related to execution context:

function test() {
  console.log(a);
  let a = 5;
}
test();

medium

A. SyntaxError due to missing semicolon

B. No error, outputs 5

C. ReferenceError because a is used before declaration

D. TypeError because a is not a function

Solution

1. Step 1: Analyze variable declaration and usage

   a is declared with let after it is logged, so it is in the temporal dead zone.

2. Step 2: Understand execution context and hoisting

   let variables are not hoisted like var, causing a ReferenceError when accessed before declaration.

3. Final Answer:

   ReferenceError because a is used before declaration -> Option C

4. Quick Check:

   Accessing let before declaration = ReferenceError [OK]

Hint: Accessing let/const before declaration causes ReferenceError [OK]

Common Mistakes:

• Thinking it outputs undefined
• Confusing with var hoisting
• Ignoring temporal dead zone

5. You want to create a block execution context to limit variable scope inside an if statement. Which code correctly creates a block execution context?

hard

A. let x = 10;
console.log(x);

B. function test() {
var x = 10;
console.log(x);
}

C. if (true)
var x = 10;
console.log(x);

D. if (true) {
let x = 10;
console.log(x);
}

Solution

1. Step 1: Identify block execution context creation

   Block execution context is created by blocks like { ... } with let or const variables.

2. Step 2: Check each option for block scope

   if (true) {
     let x = 10;
     console.log(x);
   }

   uses let inside an if block, creating a block execution context. Others use function or global scope.

3. Final Answer:

   if (true) { let x = 10; console.log(x); } -> Option D

4. Quick Check:

   Block scope needs { } and let/const [OK]

Hint: Use { } with let/const to create block execution context [OK]

Common Mistakes:

• Using var inside if block expecting block scope
• Not using braces for block
• Confusing function scope with block scope