[VL] Statements

Chapter Five: Statements in JavaScript

Introduction to Statements

• Definition of Statements:

  • A statement in JavaScript can be understood as a command that executes an action.

  • It consists of expressions, and by combining expressions with statements, you create powerful programming constructs.

  • Comparison with English Language:

  • Expressions are like words (e.g., "jump", "catch", "fish"), which evaluate to something (values).

  • Statements are like sentences (e.g., "Go to the store", "Buy a car"), which carry out a command and end with a period (or in programming, a semicolon).

Characteristics of Statements

• Key Features of Statements:

  • Statements are executed, whereas expressions are evaluated.

  • A program consists of a sequence of statements.

• Semicolon Usage:

  • Statements generally end with a semicolon (though semicolons are optional). It is recommended to always use semicolons for clarity.

Examples of Expression Statements

• Concatenation Example:

  • Example of an expression statement:

  • greeting = "Hello" + name;

    • Here, "Hello" + name is an expression that concatenates a string.

• Increment Operation:

  • counter++;

  • This statement increments the value of the variable counter by 1.

• Math Functions:

  • Example of an expression statement:

  • Math.cos(x);

    • This calculates the cosine of x.

Statement Blocks

• Definition of Statement Blocks:

  • A statement block is a grouping of statements enclosed in curly braces {} that allows multiple statements to be treated as a single statement.

• Structure of a Statement Block:

  • Example structure:

  • { x = Math.PI; cx = Math.cos(x); console.log(cx); }

  • Statements within a block are executed sequentially.

• Semicolons in Blocks:

  • Statements inside a block end with semicolons, but the block itself does not require a semicolon.

• Indentation for Readability:

  • Indentation inside blocks is not syntactically necessary, but it improves readability.

Variable Scope in JavaScript

• Insight on Scope:

  • Scope pertains to the visibility and lifecycle of variables.

  • JavaScript does not have block scope, meaning variables declared within a block will be accessible outside of it.

• Examples of Variable Visibility:

  • In traditional programming languages, variables declared inside a block would be inaccessible from outside. However, in JavaScript, variables x and cx remain accessible after the block.

Empty Statements

• Definition of Empty Statements:

  • An empty statement is represented by a semicolon (;). It serves as a placeholder where a statement is syntactically required but no action is needed.

Declaration Statements

• Types of Declaration Statements:

  • Declaration statements utilize keywords like var and function to create variables and functions (although functions will be covered later).

• Example of Variable Declaration:

  • Example:

  • var x = 2;

  • var y = Math.cos(0.75);

    • This combines declaration with assignment.

Conditional Statements

• Standard Conditional Statements:

  • Standard conditional statements involve expressions that must evaluate to true or false.

  • Syntax:

  • if (expression) { ... statements ... }

  • Example of If Statement:

  • if (username == null) { username = "John Doe"; }

• Single Statement Execution:

  • If only one statement follows the if, it doesn’t require a block.

• Negative Condition Example:

  • When checking an address condition, if (!address) verifies if address is falsy.

  • If true, it will execute the statements inside the block.

Truthy and Falsy Values

• Truthiness in JavaScript:

  • Any string can be used as a boolean expression.

  • An empty string evaluates to false, while a non-empty string evaluates to true.

  • Example to check empty string:

  • if (!address) { ... }

Else Statements

• If-Else Structure:

  • Compares the if expression and executes corresponding statements.

  • Example:

  • if (n == 1) { console.log("You have one message."); } else { console.log("You have " + n + " messages."); }

Loop Constructs

• Loop Types Discussed:

  • While Loop: Executes as long as a condition is true.

  • Do-While Loop: Similar, but checks condition after the loop runs once.

  • For Loop: A more traditional way to iterate over a range.

• Break and Continue Statements:

  • The break statement allows immediate exit from the loop.

  • The continue statement skips the rest of the current iteration and moves to the next iteration of the loop.

  • Example of a break:

    • if (i == 5) { break; }

  • Example of a continue:

    • if (someCondition) { continue; }

Conclusion

• The chapter provides insight into JavaScript statements, variable scope, conditionals, loops, and how these constructs interact with the flow of a program.

• Understanding these fundamentals is crucial for building logical structures in JavaScript programming.