JavaScript
JavaScript Variables
In JavaScript, the data is stored in the form of variables. Use the following ways to declare JavaScript variables:
- Automatically
- Using var
- Using let
- Using const
Always declare JavaScript variables before using them to avoid programming issues.
JavaScript Variable | When to Use? |
Automatically | Once used, values will ALWAYS be declared automatically. |
var | Use it ONLY when writing code for older browsers. |
let | ONLY use when const can’t be used |
const | Used when values should be kept the same |
JavaScript Identifiers
The identifiers are the unique names of JavaScript variables.
- It can be a short name or a letter like x, y, or z., etc.
- Or it can be a full descriptive name, such as sum, height, age, etc.
JavaScript identifiers are not named randomly. The rules for naming JavaScript identifiers are:
- Letters, numbers, underscores, and dollar signs can be used in an identifier’s name.
- Should ALWAYS start with a letter or sign.
- If the SAME letter is used in upper and lowercase, it will be two different identifiers.
- No identifier should be named like JavaScript keywords.
The Assignment Operator
JavaScript values are different from algebra. In JavaScript, the equal sign = is used to assign values. Therefore, an equal sign in JavaScript is always an assignment operator and never an equal to operator. It is written as a double equal sign “==”
JavaScript Data Types
A JavaScript variable value can be a number or a text. In programming languages, the text values are text strings. Here’s how data is written in JavaScript:
- When writing numbers or digits, no quotation marks are used.
- NEVER write text values without single or double quotes.
- If numbers are written inside quotation marks, they will no longer be numbers and become a text string for JavaScript.
Declaring a JavaScript Variable
In programming, a variable is declared only if it is created. To declare these variables in JavaScript var and let keywords are used. For example,
- var subjectName or
- let subjectName
A variable with no assigned values is undefined. So, to assign the value to the variables mentioned above, we will use an equal sign.
subjectName = “English”
Or it can be written as var subjectName = “English” or let subjectName = “English”.
The output value is inserted into the id= “demo” HTML paragraph after the variable is declared.
Pro Tip: Always declare variables at the beginning of the code script.
One Statement, Many Variables
Declaring a single variable in one statement is not a compulsion. Many variables can be declared together in one statement. Use the let keyword to declare the variable and separate them all with commas in between.
Value = undefined
In programming, even if the value of a variable is not defined, it will still be declared. It will be considered a variable with an undefined value.
Re-Declaring JavaScript Variables
Variables can be re-declared without losing their value with the var keyword.
However, variables declared with keywords “let” and “const” can not be re-declared.
JavaScript Arithmetic
Arithmetic statements in JavaScript are solved using the equal = and sum + operators.
Numbers, text, and signs can all be used in JavaScript arithmetic.
JavaScript Dollar Sign $
In JavaScript, the dollar sign is a letter, not a sign. So, a valid variable name may contain a dollar sign.
Professional programmers usually use this sign when defining main functions in a JavaScript library.
JavaScript Underscore (_)
Like the dollar sign $, the underscore is also treated as a letter in JavaScript. Therefore, it can be used in JavaScript identifier names. Also, a variable name with an underscore is considered a valid name.
Professional programmers use an underscore for private or hidden variables.
