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Strings & Numbers Overview

  • This lesson introduces string and number data types in JavaScript.

  • Introduction to Strings & Numbers

    In JavaScript, Strings and Numbers are two of the most commonly used data types.

    They are used in almost every application:

    • User input

    • Form validation

    • Calculations

    • Displaying messages

    • Processing data

    Understanding how strings and numbers work is fundamental to mastering JavaScript.

    In this lesson, we will cover:

    1. Overview of Strings & Numbers

    2. String Methods

    3. String Immutability

      Overview of Strings & Numbers

      What is a String ?

      A string is a sequence of characters used to represent text.

      Strings are written inside:

      • Single quotes ' '

      • Double quotes " "

      • Backticks ` `

    Different Ways to Declare Strings in JavaScript

    Shows how to create strings using double quotes, single quotes, and template literals 

    let name = "Rahul";
let city = 'Ahmedabad';
let message = `Welcome to JavaScript`;

    • All three are valid string declarations.

      What Can a String Contain ?

      A string can contain:

      • Letters

      • Numbers

      • Symbols

      • Spaces

    What Can Be Stored Inside a String

    A string can hold letters, numbers, symbols, and spaces 

    let info = "Room No. 303";

    • What is a Number ?

      A number represents numeric values used for:

      • Counting

      • Calculations

      • Comparisons

      • Mathematical operations

      JavaScript does not differentiate between integers and decimals.

    Understanding Numbers in JavaScript

    Numbers represent numeric values used for calculations and comparisons 

    let age = 25;
let price = 99.50;
let temperature = -5;
    • All are treated as number type.

    Checking Data Types Using typeof Operator

    Uses typeof to identify the data type of variables 

    let text = "Hello";
let num = 10;

console.log(typeof text); // string
console.log(typeof num);  // number

    • Strings vs Numbers

      FeatureStringNumber
      PurposeText dataNumeric data
      Quotes requiredYesNo
      Example"Hello"10
      OperationsConcatenationArithmetic

    String and Number Addition Confusion

    Shows how JavaScript concatenates string and number instead of adding them 

    let a = "10";
let b = 5;

console.log(a + b); // "105"

    • Reason: "10" is a string, so JavaScript performs string concatenation, not addition.

      String Methods

      What Are String Methods ?

      String methods are built-in functions that allow us to:

      • Manipulate text

      • Search inside strings

      • Modify string values

      • Extract parts of a string

        Commonly Used String Methods

        length

      Finding String Length

      Returns the total number of characters present in a string 

      let text = "JavaScript";
console.log(text.length);
      • toUpperCase()

      Convert String to Uppercase

      Changes all characters in a string to uppercase letters 

      let name = "javascript";
console.log(name.toUpperCase());
      • toLowerCase()

      Convert String to Lowercase

      Changes all characters in a string to lowercase letters 

      let name = "HELLO";
console.log(name.toLowerCase());
      • trim()

      Remove Extra Spaces from String

      Eliminates spaces from both the start and end of a string 

      let msg = "  Hello World  ";
console.log(msg.trim());
      • includes()

      Check Substring Presence in String

      Determines whether a string contains a specific sequence of characters 

      let text = "Learning JavaScript";

console.log(text.includes("Java"));
      • indexOf()

      Find Position of Character in String

      Returns the index of the first occurrence of a character or substring 

      let text = "JavaScript";

console.log(text.indexOf("S"));
      • slice()

      Extract Substring Using Slice Method

      Retrieves a portion of a string based on start and end index 

      let text = "JavaScript";
console.log(text.slice(0, 4));
      • replace()

      Replace Text Within a String

      Substitutes a specified value with another inside a string 

      let text = "Hello World";
console.log(text.replace("World", "JavaScript"));

      • String Immutability

        What is Immutability ?

        Immutability means:

        Once a string is created, it cannot be changed.

        JavaScript strings are immutable.

      Strings Cannot Be Modified Directly

      Demonstrates that strings are immutable and cannot be changed by index 

      let str = "Hello";
str[0] = "Y";

console.log(str); // "Hello"

      • The string does not change.

        Why Does This Happen ?

        Because strings in JavaScript:

        • Cannot be modified character by character

        • Always create a new string when changed

      String Methods Do Not Modify Original Value

      Shows that string methods return new values and do not change the original string 

      // Example: Method Does Not Change Original String
let text = "hello";

text.toUpperCase();
console.log(text);


// Correct Way to Update a String
let text = "hello";

text = text.toUpperCase();
console.log(text);

      • Why Are Strings Immutable ?

        • Better performance

        • Improved memory management

        • Predictable behavior

        • Security benefits

          String Immutability vs Arrays

          FeatureStringArray
          MutableNoYes
          Can change indexNoYes
          Methods modify originalNoSome methods do

          Common Mistakes

          • Expecting string methods to change original value

          • Trying to update string characters directly

          • Forgetting to store returned string

          Best Practices

          • Always assign the result of string methods

          • Understand immutability clearly

          • Avoid unnecessary string re-creation

          • Use meaningful variable names

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