JavaScript Basics

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Questions and Answers

Which of the following is a characteristic of JavaScript?

  • Database management
  • Server-side scripting only
  • Operating system development
  • Client-side scripting (correct)

In what year was JavaScript created?

  • 2005
  • 2000
  • 1990
  • 1995 (correct)

What name was JavaScript originally released under?

  • ECMAScript
  • JScript
  • Java
  • LiveScript (correct)

What standard was JavaScript standardized as in 1997?

<p>ECMAScript (B)</p> Signup and view all the answers

Which of the following can be used to include JavaScript in an HTML document?

<p><code>&lt;script&gt;</code> tag (A)</p> Signup and view all the answers

Which of the following is generally considered the best location for placing <script> tags in HTML?

<p>End of the <code>&lt;body&gt;</code> section (C)</p> Signup and view all the answers

Which property is used to insert dynamic content into an HTML element using Javascript?

<p>innerText (A)</p> Signup and view all the answers

What are JavaScript statements composed of?

<p>Keywords, operators, values, and expressions (B)</p> Signup and view all the answers

What term is used for values stored inside a JavaScript variable?

<p>Data (B)</p> Signup and view all the answers

Which keyword is used to declare a variable whose value cannot be changed after it is assigned?

<p>const (B)</p> Signup and view all the answers

JavaScript code can only be included directly in an HTML document.

<p>False (B)</p> Signup and view all the answers

The innerText property is used to insert text into an HTML element in Javascript.

<p>True (A)</p> Signup and view all the answers

In Javascript, ECMAScript is the official standard name for Javascript.

<p>True (A)</p> Signup and view all the answers

Microsoft created JavaScript.

<p>False (B)</p> Signup and view all the answers

A JavaScript statement is a series of instructions generally executed in top-to-bottom order.

<p>True (A)</p> Signup and view all the answers

JavaScript ignores tabs and spaces.

<p>True (A)</p> Signup and view all the answers

The const keyword is used to declare variables that can be reassigned.

<p>False (B)</p> Signup and view all the answers

Variable names cannot contain numbers.

<p>False (B)</p> Signup and view all the answers

Functions in Javascript must have a name.

<p>False (B)</p> Signup and view all the answers

The parenthesis operator () calls the function.

<p>True (A)</p> Signup and view all the answers

Given that JavaScript is a loosely typed language, what is the primary implication of this characteristic?

<p>Variables can change data type during the script execution. (A)</p> Signup and view all the answers

Which of the following code snippets demonstrates the correct usage of parseInt() to convert a string to an integer?

<p><code>parseInt(&quot;42&quot;)</code> (B)</p> Signup and view all the answers

What is the significance of function hoisting in JavaScript?

<p>It allows function declarations to be called before they are defined. (C)</p> Signup and view all the answers

In JavaScript, what is the primary difference between == and ===?

<p><code>==</code> compares values but allows type coercion, while <code>===</code> compares values and data types without type coercion. (D)</p> Signup and view all the answers

What will be the output of console.log(typeof(42 + ''))?

<p><code>string</code> (A)</p> Signup and view all the answers

Which of the following best describes the concept of lexical scope in JavaScript?

<p>The scope of a variable is determined by its location within the source code. (A)</p> Signup and view all the answers

Given the following code:

let x = 10;
function modifyX() {
  x = 20;
}
modifyX();
console.log(x);

What will be logged to the console?

<p><code>20</code> (A)</p> Signup and view all the answers

Which of the following is NOT a valid use case for comments in JavaScript?

<p>Defining variable names. (D)</p> Signup and view all the answers

What is the purpose of the return statement in a JavaScript function?

<p>To end the execution of the function and specify a value to be returned to the caller. (C)</p> Signup and view all the answers

Given the code snippet let result = '10' + 5;, what is the value and type of result?

<p>Value: <code>105</code>, Type: <code>string</code> (A)</p> Signup and view all the answers

In JavaScript, function names can only contain letters and digits, excluding underscores and dollar signs.

<p>False (B)</p> Signup and view all the answers

ECMAScript is not still commonly referred to as JavaScript.

<p>False (B)</p> Signup and view all the answers

In JavaScript, the typeof keyword can be used to determine the initial data type of a variable, but not after it has been changed later in the script.

<p>False (B)</p> Signup and view all the answers

JavaScript is exclusively a server-side scripting language to enhance web page responsiveness.

<p>False (B)</p> Signup and view all the answers

In JavaScript, the const keyword is used to declare variables that can be reassigned new values throughout the script.

<p>False (B)</p> Signup and view all the answers

Including JavaScript in the <head> section is the best practice to ensure optimal page load times for scripts.

<p>False (B)</p> Signup and view all the answers

In Javascript, '42' + 8 will result in the numerical value 50.

<p>False (B)</p> Signup and view all the answers

Whitespace such as spaces, tabs, and line breaks are significant in JavaScript and can affect code execution.

<p>False (B)</p> Signup and view all the answers

Semicolons are mandatory in all modern Javascript code, even if there is only a single statement on each line.

<p>False (B)</p> Signup and view all the answers

If the conversion of a string to a number using parseInt() or parseFloat() fails, JavaScript returns Null.

<p>False (B)</p> Signup and view all the answers

Flashcards

What is JavaScript?

A scripting language embedded in web browsers that enables client-side scripting, improving web page responsiveness.

How to include JavaScript?

To use JavaScript code in an HTML document, embed it between <script> tags.

What is function?

A named block of JavaScript code designed to perform a particular task, and it returns a final single value when invoked.

What are keywords?

Keywords are reserved words with special significance in JavaScript.

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What are operators?

Special characters that perform an operation on one or more operands.

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What is a Variable?

A container in which data can be stored and retrieved later.

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Keywords

Words that have special significance in the Javascript language.

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Global Scope means...

Variable that is accessible from anywhere in the code. Declared OUTSIDE of function.

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Function Hoisting

Function can be called before its declaration in the code because the javascript engine 'hoists' the declaration to the top during execution.

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Equality and Inequality Operators

Always prefer '===' and '!==' because they ensure strict equality by checking both value and data type.

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Writing to an HTML Element

Writing content dynamically into an HTML element using JavaScript.

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JavaScript Statements

Instructions composed of keywords, operators, values and expressions.

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Language basics

Syntax, keywords, operators, and built-in objects

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JavaScript is loosely typed

Variables are 'loosely typed', they can store any data type without prior declaration.

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Using 'let'

Declares variables that can be reassigned different values

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Using 'const'

Declares a constant, its value cannot be changed after it's assigned

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Anonymous functions

A function assigned to a variable without a name

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What are global variables?

Global variables are declared outside a function and are available throughout the code.

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Whitespace in Javascript

Whitespace is ignored, use it to improve readability

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parseInt()

Converts a string to an integer

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External Javascript

Using an external .js file to store javascript code, linked with the src attribute.

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Console output

Writing to the browser's Javascript console.

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Expression

Producing a value, used in calculations.

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Statement

Performing an action, like declaring variables.

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Syntax

The rules that govern the Javascript language.

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String literals

String literals are text enclosed in single or double quotes.

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Symbol

A type property identifier.

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Function Return

When Javascript encounters a return statement, the function will stop executing.

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Scope

Describes where variables are accessible

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Local Scope

Variables that are declared inside a function, only accessible in it's function.

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Ternary operator

A shorthand way to evaluate a condition. Consists of a condition, an expression if true, and an expression if false.

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let keyword

Variables declared with 'let', can be reassigned new values during code execution.

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const keyword

Declare a variable whose value cannot be changed after its initialization.

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Writing to HTML

Writing content dynamically into an HTML element using JavaScript, innerText property.

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Assignment Operators

Data assigned using =, +=, -=, *=, /=, %= operators. These perform an operation in place on the variable they affect.

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JS Data Types

String, Number, Boolean, Object, Function, Symbol, null, and undefined.

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Comparison Operators

Ensures equality by comparing value and type while >, <, >= and <=, only compare numerical values.

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Study Notes

Basics of JavaScript

  • JavaScript is an object-based scripting language embedded in web browsers such as Chrome, Edge, and Firefox.
  • JavaScript enables client-side scripting, which improves web page responsiveness.
  • Brendan Eich created JavaScript at Netscape in 1995; it was originally named LiveScript.
  • JavaScript was renamed to align with Java's popularity but unrelated to Java.
  • JavaScript allows faster, more interactive, and dynamic web experiences directly in the browser, improving upon the slow response times of server-side scripts.
  • Microsoft introduced JScript, which led to concerns about fragmentation.
  • In 1997, JavaScript was standardized as ECMAScript by Ecma International.
  • Despite standardization, most people refer to it as JavaScript.
  • Brendan Eich co-founded Mozilla and helped launch Firefox.
  • Core capabilities include language basics (syntax, keywords, operators, and built-in objects), web page interaction (DOM manipulation), and web applications (creating responsive web apps and handling JSON data).

Including Scripts

  • JavaScript code can be included directly in an HTML document or placed in external plain text files with a ".js" extension.
  • To include JavaScript code directly in an HTML document, insert it between <script> tags.
  • JavaScript code in an HTML document inline should be inserted between the opening <script> and closing </script> tags.
  • JavaScript can be placed in the <head> section, inside the <body> section, or at the end of the <body> section of an HTML document.
  • Placing JavaScript in the <head> section is not recommended unless it's a small script.
  • Placing JavaScript inside the <body> section can slow down page rendering.
  • The best practice is putting JavaScript at the end of the <body> section to ensure the page loads first before running scripts.
  • To include an external JavaScript file, use the <script> tag with the "src" attribute: <script src="external_script.js"></script>.
  • External JavaScript files promote reusability, better organization, and easier maintenance.
  • Reusability means the same script can be used across multiple web pages.
  • Better organization keeps HTML clean and separates content from behavior.
  • Easier maintenance simplifies updating the .js file updates all linked pages.
  • Do not include <script> tags inside the .js file – only write JavaScript code in the .js file.

Console Output

  • JavaScript can dynamically write content into an HTML element using the innerText property to insert text into the element with the specified ID.
  • JavaScript can display output using a pop-up alert.
  • The alert( ) method of the window object displays content specified within the parentheses in a dialog box.
  • The log( ) method of the console object displays content specified within the parentheses in a console window for debugging.
  • When developing in JavaScript, it is better to display output in the browser's JavaScript console using console.log().

Make Statements

  • JavaScript code is composed of a series of instructions called "statements" executed in top-to-bottom order.
  • Statements comprise keywords, operators, values, and expressions.
  • Keywords are words with special significance in the JavaScript language.
  • Operators are special characters that perform an operation on one or more operands.
  • Values are text strings, numbers, boolean true or false, undefined, and null.
  • Expressions are units of code that produce a single value.
  • In earlier JavaScript, every statement must end with a semicolon (;).
  • Now, semicolons are optional, unless multiple statements are on the same line.
  • Some developers still prefer semicolons for clarity.
  • JavaScript ignores extra spaces, tabs, and line breaks.
  • Spacing improves readability.
  • Use the space bar to indent statements, as tab spacing may be treated differently in text editors.
  • JavaScript statements are often grouped using curly brackets {} inside function blocks.
  • Using curly brackets within function blocks allows for reusable execution when required.
  • Indenting statements by two spaces for better readability is good practice.
  • The rules that govern the JavaScript language are called "syntax."

Values and Variables

  • JavaScript recognizes two types of values: fixed values (literals) and variable values (variables).
  • Number literals can be integer numbers (e.g., 100) or floating-point numbers (e.g., 3.142).
  • String literals are text enclosed in single ('JavaScript Fun') or double ("JavaScript Fun") quotes.
  • Maintain consistency with string literals by using either 'single' or "double" quotes.
  • Variable values, or "variables," store data within a script.
  • Variables are created using the let keyword; for example, let total creates a variable named "total".
  • Assign a value to a variable using the = assignment operator; for example, let total = 300.
  • An expression produces a single value by combining values and operators.
  • Case sensitivity means that JavaScript is case-sensitive, so total and Total are different variables.
  • Comments explain code for readability and debugging.
  • Use single-line comments (//) for short explanations.
  • Use multi-line comments (/* ... */) for longer explanations or disabling blocks of code.
  • Comments explain complex logic.
  • "Comment-out" lines of code to prevent their execution while debugging.

Storing values

  • A "variable" is a container in which data can be stored and retrieved later.
  • JavaScript variables are easier to use because they are "loosely typed, " allowing any type of data to be stored.
  • Strongly typed variables require explicit data type declaration before use.
  • Loosely typed variables do not require data type declaration.
  • JavaScript variables can be declared in four ways: automatically, using var, using let, and using const.
  • Declaring variables automatically involves assigning a value to a variable without explicitly declaring it.
  • An example of automatically declaring a variable x, y and z are undeclared; however, they are declared once there first use in an expression.
  • It is considered better practice to always declare variables before use.
  • The var keyword was used in all Javascript code prior to ECMAScript 2015.
  • The var keyword should only be used in code written for older browsers.
  • The let and const keywords were added to JavaScript in 2015.
  • Variables declared with let can be reassigned new values as the script proceeds.
  • A let variable can be declared without a value and assigned later.
  • Multiple variables may be declared on a single line too.
  • Variables declared with const are constant values and cannot be changed.
  • Use const if the type should not be changed (arrays and objects).
  • Choose meaningful names for variables to make the script easier to read.
  • JavaScript automatically sets the variable type for the value assigned.
  • The variable type can change with subsequent assignments using a different data type to the original variable type.
  • The typeof keyword reveals the current variable type.

Functions

  • A JavaScript function is a block of code designed to perform a particular task and return a final value.
  • JavaScript functions are executed when "something" invokes or calls them.
  • JavaScript functions are defined with the function keyword, followed by a name and parentheses ().
  • Function names can contain letters, digits, underscores, and dollar signs.
  • The parentheses may include parameter names separated by commas.
  • The code to be executed by the function is placed inside curly brackets: {}.
  • Function arguments are the values received by the function when invoked.
  • Inside the function, the arguments (parameters) behave as local variables.
  • Function invocation happens when an event occurs (e.g., a user clicks a button) or when called from JavaScript code.
  • Automatically invoked functions are called self-invoked.
  • When JavaScript reaches a return statement, the function stops executing.
  • If a function was invoked from a statement, JavaScript will "return" to execute the code after the invoking statement.
  • Functions compute a return value that is "returned" back to the "caller."

Assigning Functions

  • Named functions can be assigned to variables.
  • Anonymous functions (or function expressions) can be assigned to variables.
  • Self-invoking functions are used to execute a function automatically when the script loads.
  • Function declarations can be called before they are defined due to hoisting.
  • Function expressions do not support hoisting.

Scope

  • Scope determines where a variable is accessible in a script.
  • JavaScript uses lexical scope, meaning the variable's environment decides its accessibility.
  • Two primary types of scope are global scope and local scope.
  • A global variable is declared outside of any function and is accessible throughout the entire script, but may cause naming conflict issues with external scripts.
  • A local variable is declared inside a function and only exists within that function. It cannot be accessed outside.

Convert Values

  • Conversion is important because JavaScript handles different data types differently.
  • Using mixed data types (e.g., strings and numbers) in operations can lead to unexpected results.
  • Sometimes, it’s necessary to convert a string or other types to numbers perform mathematical operations.
  • Parsing examples include converting the string '42.5px' to the floating-point number 42.5.
  • parseInt() converts a string to an integer, ignoring non-numeric characters after the number.
  • When the conversion fails, JavaScript returns NaN (Not a Number).
  • You can check if a value is not a number using the isNaN(value) function, which returns true if the value is not a number.
  • Numbers can be converted to strings using the String() method or using the toString() method.
  • Example conversions include String(42) → '42' and num.toString() where num = 42, the latter returning '42'.

Arithmetic Operators

  • Arithmetical operators commonly used in JavaScript include addition/concatenation, subtraction, multiplication, division, modulus, increment, decrement, and exponentiation.

Assign Values

  • Shorthand Assignment Operators perform an operation and assign the result in one step.
  • The basic assignment operator in JavaScript is the equal (=).
  • Operators include +=, -=, *=, /=, %=, and **=.

Comparison Operators

  • Comparison operators evaluate conditions by comparing two values.
  • The === and !== operators ensure strict equality by checking both value and data type.
  • Relational operators like >, <, >=, and <= compare numerical values. This can lead to type coercion.
  • The ternary operator (?:) is a shorthand way to evaluate a condition.
  • A ternary operator involves three parts: condition ? expression_if_true : expression_if_false.

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