JavaScript Arrays and Strings for Interview Prep

Introduction

Welcome to this course! Before we delve deeper into JavaScript essentials for interview prep, let's start with some foundational JavaScript features — specifically, arrays and strings . These features allow JavaScript to group multiple elements, such as numbers or characters, under a single entity.

Revising Arrays and Strings

As our starting point, it's crucial to understand how arrays and strings function in JavaScript. An array is a built-in object that provides a way to store multiple values in a single variable and is mutable (we can change it after creation), while strings are immutable (unalterable post-creation). Let's see examples: JavaScript const myList = [1, 2, 3, 4]; let myString = "hello"; // Now let's try to change the first element of both these features // Using index to change the element at the specified index (0 in this case) myList[0] = 100; myString[0] = "t"; // does not change anything // The following method does not change the string in place, // but returns a new string where 'h' is replaced with 'H' myString.replace('h', 'H'); // So it is possible to obtain a new string like this: const newString = myString.replace('h', 'H'); console.log(myList); // prints [100, 2, 3, 4] console.log(myString); // prints hello console.log(newString); // prints Hello const myList = [1, 2, 3, 4]; let myString = "hello"; // Now let's try to change the first element of both these features // Using index to change the element at the specified index (0 in this case) myList[0] = 100; myString[0] = "t"; // does not change anything // The following method does not change the string in place, // but returns a new string where 'h' is replaced with 'H' myString.replace('h', 'H'); // So it is possible to obtain a new string like this: const newString = myString.replace('h', 'H'); console.log(myList); // prints [100, 2, 3, 4] console.log(myString); // prints hello console.log(newString); // prints Hello

Diving Into Lists

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Indexing and Common Operations

Both arrays and strings allow us to access individual elements through indexing. In JavaScript, we start counting from 0, implying the first element is at index 0, the second at index 1, and so on. Negative indexing is not directly supported in JavaScript, but you can work around it by adjusting indices based on the length of the collection.

We have many operations we can perform on our lists and strings. We can slice them, concatenate them, and even find an occurrence of a particular element!

JavaScript

const myList = [1, 2, 3, 4, 5];
const myString = "Hello";

// Slicing: `slice` for array and `substring` or `slice` for strings
// Note: The first index is inclusive and the second index is exclusive 
const sliceList = myList.slice(2, 4); // [3, 4]
const sliceString = myString.substring(1, 3); // "el"

// Concatenation: `concat` or spread operator for lists and `+` operator or template literals for strings
const concatenateList = myList.concat([6, 7, 8]); // [1, 2, 3, 4, 5, 6, 7, 8]
const concatenateListSpread = [...myList, 6, 7, 8]; // [1, 2, 3, 4, 5, 6, 7, 8]
const concatenateString = `${myString}, world!`; // "Hello, world!"
const concatenateStringPlus = myString + ", world!"; // "Hello, world!"

// Finding the index of an element in a list or a string
// `indexOf` returns the first occurrence index of the element
// returns -1 if the list or the string doesn't contain the element
const indexList = myList.indexOf(3); // 2 - Index of element '3'
const indexString = myString.indexOf('e'); // 1 - Index of character 'e'

// Sorting items in array in non-increasing order
const sortedList = [...myList].sort((a, b) => b - a); // [5, 4, 3, 2, 1]

const myList = [1, 2, 3, 4, 5];
const myString = "Hello";

// Slicing: `slice` for array and `substring` or `slice` for strings
// Note: The first index is inclusive and the second index is exclusive 
const sliceList = myList.slice(2, 4); // [3, 4]
const sliceString = myString.substring(1, 3); // "el"

// Concatenation: `concat` or spread operator for lists and `+` operator or template literals for strings
const concatenateList = myList.concat([6, 7, 8]); // [1, 2, 3, 4, 5, 6, 7, 8]
const concatenateListSpread = [...myList, 6, 7, 8]; // [1, 2, 3, 4, 5, 6, 7, 8]
const concatenateString = `${myString}, world!`; // "Hello, world!"
const concatenateStringPlus = myString + ", world!"; // "Hello, world!"

// Finding the index of an element in a list or a string
// `indexOf` returns the first occurrence index of the element
// returns -1 if the list or the string doesn't contain the element
const indexList = myList.indexOf(3); // 2 - Index of element '3'
const indexString = myString.indexOf('e'); // 1 - Index of character 'e'

// Sorting items in array in non-increasing order
const sortedList = [...myList].sort((a, b) => b - a); // [5, 4, 3, 2, 1]

When using the spread operator for concatenation (...myList), the elements of myList are expanded into individual elements and combined with additional elements [6, 7, 8].

Template literals, enclosed by backticks (`), allow you to embed expressions inside a string using ${expression}. This provides a cleaner and more readable way to concatenate strings compared to using the + operator.

The spread operator, used in [...myList], creates a shallow copy of myList, ensuring the original array remains unchanged. The sort method with a comparison function (a, b) => b - a sorts the array in non-increasing order. Here's how it works:

The comparison function receives two arguments, a and b.
If the returned value is negative, a should come before b.
If the returned value is positive, b should come before a.
If the returned value is zero, their positions are unchanged relative to each other.

By subtracting a from b, it sorts the elements in descending order (from highest to lowest). Thus, sortedList will contain [5, 4, 3, 2, 1], a sorted copy of myList in descending order.

Introduction

Revising Arrays and Strings

Diving Into Lists

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Understanding Strings

Indexing and Common Operations

Lesson Summary and Practice