Kotlin let function keyword explained with examples

The Kotlin let keyword is one of several Kotlin scope function keywords.

The scope functions keyword in Kotlin allows you to execute a block of code in the context of the object.

Using scope functions make your code more concise and readable.

For example, suppose you have a User class with the following definitions:

class User(var username: String, var isActive: Boolean) {
    fun hello() {
        println("Hello! I'm $username")
    }
}

The usual way of instantiating a User object would be to declare a new variable as shown below:

val user = User("Nathan", true)
user.hello() // Hello! I'm Nathan

user.username = "Jack"
user.hello() // Hello! I'm Jack

By using the let keyword, you can omit assigning the returned User object to the variable.

The code example below has the same output as the one above:

User("Nathan", true).let {
    it.hello() // Hello! I'm Nathan

    it.username = "Jack"
    it.hello() // Hello! I'm Jack
}

Instead of having to initialize a new variable, you can call the methods and properties of the generated User object directly within the let keyword using a lambda expression.

The object where you call the let function can be accessed inside let by using the it keyword:

User("Nathan", true).let {
    println(it.username) // Nathan
    println(it.isActive) // true
}

The let keyword returns the result of the lambda expression you defined inside the code block.

In the above example, the lambda expression returns nothing, so it returns a Unit type:

val unit = User("Nathan", true).let {
    it.hello() // Hello! I'm Nathan

    it.username = "Jack"
    it.hello() // Hello! I'm Jack
}

println(unit is Unit) // true

To return something other than Unit, the last code line inside your let code block must return a value.

For example, you can return the length value of the username property:

val usernameLength = User("Nathan", true).let {
    it.hello()
    it.username = "Jack"
    it.username.length
}

println(usernameLength) // 4

if it.username.length above is not on the last line of the code block, let would still return a Unit instead of an Int value.

Finally, the let keyword can also be called on any Kotlin built-in objects like a String or a Boolean.

This is particularly useful when you have a nullable type variable. You need to perform a safe call with ?. symbol only once:

val name: String? = "Nathan"

name?.let {
    println(it.length) // 6
    println(it.lastIndex) // 5
}

Without using let, you need to add the safe call symbol ?. before the call to length and lastIndex properties as follows:

val name: String? = "Nathan"

println(name?.length)
println(name?.lastIndex)

Now you’ve learned how the let keyword works in Kotlin. Very nice! 👍