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:

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.

Example:

var x;
console.log(x);
x = "123";
console.log(x);
var x;
console.log(x);

Result:

undefined
123
123