JS Variables

1. Intro

  • All JavaScript variables must be identified with unique names. These unique names are called identifiers.
  • In JavaScript, objects and functions are also variables.

2. Identifiers 變數命名規則

  • The first character must be:

    • a letter
    • an underscore (_)
    • a dollar sign ($)
  • Subsequent characters may be:

    • letters
    • digits
    • underscores (_)
    • dollar signs ($)
    • PS: Hyphens are not allowed in JavaScript. It is reserved for subtractions.

Do not start names with a $ sign.
It will put you in conflict with many JavaScript library names.

  • JavaScript and Camel Case(駝峰式)
    • JavaScript programmers tend to use camel case that starts with a lowercase letter:

      EX: firstName

    • Historically, programmers have used different ways of joining multiple words into one variable name:
Way Example
Hyphens first-name
Underscore first_name
Upper Camel Case (Pascal Case) FirstName
Lower Camel Case firstName

3. Declaring JavaScript Variables

  • You declare a JavaScript variable with the var keyword:
  • It's a good programming practice to declare all variables at the beginning of a script.
var carName1; //A variable declared without a value will have the value **undefined**.
var carName2 = "Volvo";
var person = "John Doe", carName3 = "Volvo", price = 200;

4. Re-Declaring JavaScript Variables

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


var x;
x = "123";
var x;



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