What is the difference between `let` and `var` in swift?

Question

What is the difference between let and var in Apple\'s Swift language?

In my understanding, it is a compiled language but it does not check

Answer 1

let defines a "constant". Its value is set once and only once, though not necessarily when you declare it. For example, you use let to define a property in a class that must be set during initialization:

class Person {

    let firstName: String
    let lastName: String

    init(first: String, last: String) {
         firstName = first
         lastName = last
         super.init()
    }
}

With this setup, it's invalid to assign to firstName or lastName after calling (e.g.) Person(first:"Malcolm", last:"Reynolds") to create a Person instance.

You must define a type for all variables (let or var) at compile time, and any code that attempts to set a variable may only use that type (or a subtype). You can assign a value at run time, but its type must be known at compile time.