Array creation in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the simplest way to create an empty array in JavaScript?

You can create an empty array using square brackets: []. This makes a new list that can hold items.

beginner

How do you create an array with initial values 1, 2, and 3?

Use square brackets with the values inside separated by commas: [1, 2, 3]. This creates an array holding those numbers.

intermediate

What does new Array(5) do in JavaScript?

It creates an array with 5 empty slots. The array has length 5 but no actual values set yet.

intermediate

How can you create an array from a string like 'hello'?

Use Array.from('hello'). This makes an array of each character: ['h', 'e', 'l', 'l', 'o'].

intermediate

What is the difference between [] and new Array()?

[] creates an empty array. new Array() also creates an empty array but can behave differently if you pass a number (it sets length).

Which syntax creates an array with three elements: 10, 20, and 30?

A All of the above

B [10, 20, 30]

C Array(10, 20, 30)

D new Array(10, 20, 30)

All these syntaxes create an array with elements 10, 20, and 30.

What does new Array(4) create?

A An array with 4 undefined elements

B An error

C An array with one element: 4

D An array with length 4 but empty slots

new Array(4) creates an array with length 4 but the slots are empty, not set to undefined.

How to create an array from the string 'abc'?

A Array.from('abc')

B ['abc']

C new Array('abc')

D String.toArray('abc')

Array.from('abc') creates ['a', 'b', 'c'].

Which is the correct way to create an empty array?

A []

B Both A and B

C new Array()

D Array.empty()

Both [] and new Array() create empty arrays.

What happens if you do new Array(3, 4)?

A Creates an array with length 4

B Creates an array with length 3

C Creates an array with elements 3 and 4

D Throws an error

new Array(3, 4) creates an array with two elements: 3 and 4.

Explain how to create arrays in JavaScript and the difference between using [] and new Array().

Think about how length and elements are set.

Describe how to convert a string into an array of characters in JavaScript.

Consider how strings and arrays relate.

Practice

1. Which of the following is the correct way to create an empty array in JavaScript?

easy

A. const arr = [];

B. const arr = ();

C. const arr = {};

D. const arr = '';

Solution

1. Step 1: Understand array syntax

   In JavaScript, arrays are created using square brackets [].

2. Step 2: Identify correct empty array creation

   Only [] creates an empty array; parentheses, braces, or quotes create other types.

3. Final Answer:

   const arr = []; -> Option A

4. Quick Check:

   Empty array = [] [OK]

Hint: Remember arrays use square brackets [] to hold items [OK]

Common Mistakes:

• Using parentheses () instead of brackets
• Using curly braces {} which create objects
• Using quotes '' which create strings

2. Which of these is the correct syntax to create an array with three numbers: 1, 2, and 3?

easy

A. const numbers = (1, 2, 3);

B. const numbers = [1, 2, 3];

C. const numbers = {1, 2, 3};

D. const numbers = '1, 2, 3';

Solution

1. Step 1: Recognize array literal syntax

   Arrays use square brackets [] with comma-separated values inside.

2. Step 2: Check each option

   Only const numbers = [1, 2, 3]; uses square brackets with numbers separated by commas.

3. Final Answer:

   const numbers = [1, 2, 3]; -> Option B

4. Quick Check:

   Array with 3 numbers = [1, 2, 3] [OK]

Hint: Arrays always use square brackets with commas [OK]

Common Mistakes:

• Using parentheses which create expressions, not arrays
• Using curly braces which create objects
• Using quotes which create strings

3. What will be the output of the following code?

const arr = [10, 20, 30];
console.log(arr.length);

medium

A. undefined

B. 30

C. Error

D. 3

Solution

1. Step 1: Understand array length property

   The length property returns the number of elements in the array.

2. Step 2: Count elements in the array

   The array has 3 elements: 10, 20, and 30, so arr.length is 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Array length = 3 [OK]

Hint: Array length counts items, not values [OK]

Common Mistakes:

• Confusing length with last element value
• Expecting length to be undefined
• Thinking length causes an error

4. Find the error in this array creation code:

const fruits = ["apple", "banana", "orange";

medium

A. Missing comma between elements

B. Strings should use single quotes instead of double quotes

C. Missing closing square bracket ]

D. Array elements must be numbers only

Solution

1. Step 1: Check array syntax

   Arrays must start with [ and end with ]. The code misses the closing bracket.

2. Step 2: Verify other parts

   Double quotes are allowed for strings, commas are present between elements.

3. Final Answer:

   Missing closing square bracket ] -> Option C

4. Quick Check:

   Array syntax requires matching brackets [OK]

Hint: Count opening and closing brackets to spot errors [OK]

Common Mistakes:

• Forgetting to close the array with ]
• Thinking quotes style causes error
• Assuming arrays only hold numbers

5. You want to create an array that contains the numbers from 1 to 5 using a loop. Which code correctly creates this array?

hard

A. const arr = [];
for(let i = 0; i < 5; i++) {
arr[i] = i + 1;
}

B. const arr = [];
for(let i = 1; i <= 5; i++) {
arr[i] = i;
}

C. const arr = new Array(5);
for(let i = 1; i <= 5; i++) {
arr[i] = i;
}

D. const arr = [];
for(let i = 1; i < 5; i++) {
arr.push(i);
}

Solution

1. Step 1: Understand array creation with loop

   We want numbers 1 to 5, so loop must run 5 times and add values 1 through 5.

2. Step 2: Analyze each option

   const arr = []; for(let i = 1; i <= 5; i++) { arr[i] = i; } assigns values starting at index 1, leaving index 0 empty. const arr = new Array(5); for(let i = 1; i <= 5; i++) { arr[i] = i; } creates array of length 5 but assigns starting at index 1, leaving index 0 empty. const arr = []; for(let i = 0; i < 5; i++) { arr[i] = i + 1; } assigns values 1 to 5 at indexes 0 to 4 correctly. const arr = []; for(let i = 1; i < 5; i++) { arr.push(i); } loops only to 4, missing 5.

3. Step 3: Choose best correct code

   const arr = []; for(let i = 0; i < 5; i++) { arr[i] = i + 1; } correctly fills array indexes 0 to 4 with values 1 to 5.

4. Final Answer:

   const arr = []; for(let i = 0; i < 5; i++) { arr[i] = i + 1; } -> Option A

5. Quick Check:

   Loop 0-4, assign i+1 = 1-5 [OK]

Hint: Loop from 0 to length-1, assign i+1 for 1 to 5 [OK]

Common Mistakes:

• Starting loop at 1 but assigning to arr[i] leaves arr[0] empty
• Loop ending before 5
• Using new Array(5) without filling all indexes