Let and const

In ES5 JavaScript, we use the var keyword to define variables. In most cases, var can simply be replaced with let, with the major difference between the two constructs being the scoping of the variable with respect to blocks. The following example from MDN web docs, or previously Mozilla Developer Network (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let), demonstrates the subtle difference between the two:

function varTest() {
  var x = 1;
  if (true) {
    var x = 2;  // same variable!
    console.log(x);  // 2
  }
  console.log(x);  // 2
 }
 
 function letTest() {
  let x = 1;
  if (true) {
    let x = 2;  // different variable
    console.log(x);  // 2
  }
  console.log(x);  // 1
 }

So, while you must use additional caution in cases like the preceding one, in most cases you can simply replace var with let.

The const keyword, unlike let, defines a variable as a constant; that is, you cannot reassign a variable initialized with const at a later date. For example, the following code causes an error with a message similar to invalid assignment to const a:

const a = 1;
a = 2;

On the other hand the same code, using var or let to define a, would run successfully.

Note that if a is an object, you are allowed to modify object properties of a.

The following code will run successfully:

const obj = {};
obj.name = ‘My Object’;

However, attempting to redefine objects such as in obj = {name: “other object”} would cause an error.

I find that in most programming contexts, const is typically more appropriate than let, as most variables you use never need to be redefined. My recommendation is to use const as much as you can, and use let only when you have a reason to redefine the variable later.