JavaScript Variables

المعرفة:: JavaScript الحالة::مؤرشفة المراجع:: JavaScript Essential Training

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Variables and Data Types

• Naming Objects: The creation of containers to put objects within.
• Variable: A container with some piece of data.

var

The var statement declares a function-scoped or globally-scoped variable, optionally initializing it to a value.

var container = 5; // mutable
container = "red";
var x = 4,
  y = 5,
  z = "blue";
var empty;

let

The let statement declares a block-scoped local variable, optionally initializing it to a value.

var color = "purple";
 
document.querySelector(".left").style.backgroundColor = color;
document.querySelector(".left .color-value").innerHTML = color;
 
color = "skyblue";
 
function headingColor() {
  let color = "blue"; // scoped
  document.querySelector(".title").style.color = color;
}
 
headingColor();
 
document.querySelector(".right").style.backgroundColor = color;
document.querySelector(".right .color-value").innerHTML = color;

Which to use?

• For most situations, when you want to use a changeable or mutable variable, you should use a let.
• That includes when you want to use it in global scope, because when you declare a let in global scope, it will apply everywhere except where you re-declare it.
• The var is only really useful if you want a mutable variable with global scope all the time, but that situation is quite rare and is a special case.
• So the default is, when you want a changeable or mutable variable use a let.

const

Constants are block-scoped, much like variables declared using the let keyword. The value of a constant can’t be changed through reassignment, and it can’t be re-declared. However, if a constant is an object or array, its properties or items can be updated or removed.

const number = 42;