Let and Const

You are probably familiar with declaring variables in JavaScript using a syntax such as:

var name = "Shane";

You can now declare variables using two new keywords let and const. The main difference has to do with how scope works with these variables.

• var is function scoped
• let is block scoped

For instance, look at the code below using var.

var myArray = [1,2,3];
for (var i = 0; i < myArray.length; i++) {
  var doubleNumber = myArray[i] * 2;
}
console.log(doubleNumber); // You would expect this to be undefined, but it's 6.

If we change doubleNumber to be defined using let you get a different result:

var myArray = [1,2,3];
for (var i = 0; i < myArray.length; i++) {
  let doubleNumber = myArray[i] * 2;
}
console.log(doubleNumber); // Undefined.

const and let are almost exactly the same, except when you use const you are saying that the value is not going to be redefined.

Note: If your const variable is an object, you can still change the values of the object, but you can't reassign it to a different type.

I typically use const unless there is a specific reason to use let (i.e. it's a primitive variable type that you will be changing). You should not need to use var.