In this first example, x , y , and z are undeclared variables.
They are automatically declared when first used:
It is considered good programming practice to always declare variables before use.
From the examples you can guess:
The var keyword was used in all JavaScript code from 1995 to 2015.
The let and const keywords were added to JavaScript in 2015.
The var keyword should only be used in code written for older browsers.
The two variables price1 and price2 are declared with the const keyword.
These are constant values and cannot be changed.
The variable total is declared with the let keyword.
The value total can be changed.
2. Always use const if the value should not be changed
3. Always use const if the type should not be changed (Arrays and Objects)
4. Only use let if you can't use const
5. Only use var if you MUST support old browsers.
Just like in algebra, variables hold values:
let x = 5;Just like in algebra, variables are used in expressions:
let z = x + y;From the example above, you can guess that the total is calculated to be 11.
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:
JavaScript identifiers are case-sensitive.
In JavaScript, the equal sign ( = ) is an "assignment" operator, not an "equal to" operator.
This is different from algebra. The following does not make sense in algebra:
In JavaScript, however, it makes perfect sense: it assigns the value of x + 5 to x.
(It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.)
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.
JavaScript can handle many types of data, but for now, just think of numbers and 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:
var carName; let carName;After the declaration, the variable has no value (technically it is undefined ).
To assign a value to the variable, use the equal sign:
carName = "Volvo";You can also assign a value to the variable when you declare it:
let carName = "Volvo";In the example below, we create a variable called carName and assign the value "Volvo" to it.
Then we "output" the value inside an HTML paragraph with >
let carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
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:
A declaration can span multiple lines:
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 .
The variable carName will have the value undefined after the execution of this statement:
If you re-declare a JavaScript variable declared with var , it will not lose its value.
The variable carName will still have the value "Volvo" after the execution of these statements:
You cannot re-declare a variable declared with let or const .
This will not work:
let carName = "Volvo";As with algebra, you can do arithmetic with JavaScript variables, using operators like = and + :
You can also add strings, but strings will be concatenated: