Rule of thumb: start with const, if you need to change the value later, change to let, and forget about var. More on reddit.com

Using the function keyword creates a function declaration. Function declarations are hoisted to the top of the scope, thus the following code works fine: add(1, 2) function add (a, b) { return a + b } There are readability advantages to using the function keyword as it clearly communicates that this is a function. We can also create an anonymous function expression and, because functions in JavaScript are first class citizens, assign a function to a named variable with let, const or var. Keep in mind though, variables declared with let and const are hoisted, but not initialized with a value. Whereas variables declared with var are hoisted and initialized with the value undefined. So because add is invoked prior to it's initialization in the code below we encounter an error. With let/const: add(1, 2) // Reference error - cannot access 'add' before initialization const add = function (a, b) { return a + b } With var: add(1, 2) // Type error - 'add' is not a function (as it is initialized as undefined when hoisted) var add = function (a, b) { return a + b } If you're assigning a function to a variable I can't think of any situation where you wouldn't use const. Functions should always be pure and immutable whenever possible. Given a certain input, the output should always be consistently the same. Using const protects you from accidentally reassigning a variable containing a function expression. Compare this to use of the function keyword, which allows you to declare a named function twice, overiding the previous: function add (a, b) { return a + b } function add (a, b) { return a - b } add(2, 1) // 1 This is a disadvantage of function declarations versus expressions. Provided you understand the pros/cons and how hoisting works, it is really personal choice whether you prefer function declarations versus expressions. More on reddit.com