JavaScript Basics

• JavaScript is a scripting language that enables you to create dynamically updating content, control multimedia, animate images, and much more.
• The core client-side JavaScript language consists of some common programming features that allow you to do things like:
  • Store useful values inside variables.
  • Operations on pieces of text (known as “strings” in programming).
  • Running code in response to certain events occurring on a web page.

JavaScript Variables

• Variables are containers for storing data (storing data values).
• There are 4 ways to declare a JS Variable:
  • Using var
  • Using let
  • Using const
  • Using nothing
• The var keyword is used in all JavaScript code from 1995 to 2015.
• The let and const keywords were added to JavaScript in 2015.
• If you want your code to run in older browser, you must use var.
• If you want a general rule: always declare variables with const.
• If you think the value of the variable can change, use let.
• Just like in algebra, variables hold values
• Just like in algebra, variables are used in expressions

• Variables are containers for storing values.

• All JavaScript variables must be identified with unique names. These unique names are called identifiers.
  • Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).
• The general rules for constructing names for variables (unique identifiers) are:
  • Names can contain letters, digits, underscores, and dollar signs.
  • Names must begin with a letter
  • Names can also begin with $ and _ (but we will not use it in this tutorial)
  • Names are case sensitive (y and Y are different variables)
  • Reserved words (like JavaScript keywords) cannot be used as names
• In JavaScript, the equal sign (=) is an “assignment” operator, not an “equal to” operator.

• The “equal to” operator is written like == in JavaScript.

• JavaScript variables can hold numbers like 100 and text values like “John Doe”.
• In programming, text values are called text strings.

• Strings are written inside double or single quotes. Numbers are written without quotes. If you put a number in quotes, it will be treated as a text string.

• Creating a variable in JavaScript is called “declaring” a variable.
• You declare a JavaScript variable with the var or the let keyword
• After the declaration, the variable has no value (technically it is undefined).
• To assign a value to the variable, use the equal sign
• You can also assign a value to the variable when you declare it
• It’s a good programming practice to declare all variables at the beginning of a script.
• You can declare many variables in one statement. Start the statement with let and separate the variables by comma.

Value = undefined:

• In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input.
• A variable declared without a value will have the value undefined

Re-declaring JS Variables

• If you re-declare a JavaScript variable declared with var, it will not lose its value.

• You cannot re-declare a variable declared with let or const.

• Since JavaScript treats a dollar sign as a letter, identifiers containing $ are valid variable names. Using the dollar sign is not very common in JavaScript, but professional programmers often use it as an alias for the main function in a JavaScript library.
• Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names. Using the underscore is not very common in JavaScript, but a convention among professional programmers is to use it as an alias for “private (hidden)” variables.

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