let adder = () => {
if (sum === undefined) {
var sum = 0
}
sum = sum + 10
console.log(sum)
if (sum >= 100) {
return sum
}
adder()
}Please explain why the sum = 0 every time the recursive function is being activated? Why doesn't it accumulate?
How about utilising the closure property?
const adder = (sum = 0) =>
() => ++sum
const a = adder()
const b = adder()
console.log(a(), a(), a()) // 1 2 3
console.log(b(), b(), b()) // 1 2 3
console.log(a(), a(), a()) // 4 5 6
console.log(b(), b(), b()) // 4 5 6Or give the adder a parameter -
const adder = (sum = 0) =>
(n = 1) => sum += n
const a = adder()
const b = adder()
console.log(a(10), a(10), a(10)) // 10 20 30
console.log(b(), b(), b()) // 1 2 3
console.log(a(), a(), a()) // 31 32 33
console.log(b(), b(), b()) // 4 5 6Variables in javascript are always scoped inside the function they're called from. If you define a sum variable inside a function and call a function, even if it's the same function, that function will not have access to the variable. You can, however, define a global variable, and the inner function will have access to it. However, this is not recommended and goes against the concept of recursion.
Solution 1 (global variable, not recommended):
// note how this is declared outside of any function
var sum;
let adder = () => {
if (sum === undefined) {
// the var here is gone to avoid redeclaring at a local scope
sum = 0
}
sum = sum + 10
console.log(sum)
if (sum >= 100) {
return sum
}
// we also return the value
return adder()
}
Solution 2 (proper recursion, recommended):
// note how sum is an argument now
let adder = sum => {
if (sum === undefined) {
sum = 0
}
sum = sum + 10
console.log(sum)
if (sum >= 100) {
return sum
}
// we pass sum in as an argument and return it
return adder(sum)
}
adder call returns 100, but that's not being returned by the previous adder calls, so it just defaults to undefined.
var sumgets hoisted to the top of the function, so your function is effectivelyvar sum; if( sum === undefined) sum = 0;which is always true.