Lazy evaluation in JavaScript

lazy-evaluation.js

function Let(f) {
  let called = false
  let v = null
  return function () {
    if(called) return v
    console.log('evaluating.')
    v = f()
    called = true
    return v
  }
}

const n = Let(function() { return 23 })

console.log(n())

console.log(n() + 42)

console.log(n() - 4)

A close inmplementation in Ruby

class Let
  attr_accessor :called, :v, :func

  def initialize(&func)
    @called = false
    @v = nil
    @func = func
  end

  def call
    if @called
      @v
    else
      @called = true
      @v = @func.call
    end
  end
end

p = Let.new do
  13
end
# => #<Let:0x00007f810a87b3a0 @called=false, @func=#<Proc:0x00007f810a87b378@(pry):19>, @v=nil>

p.call # => 13
p # => #<Let:0x00007f810a87b3a0 @called=true, @func=#<Proc:0x00007f810a87b378@(pry):19>, @v=13>