What makes (_=1)=>$=>_++ act as a tally?

Question

What makes (_=1)=>$=>_++ act as a tally?

One of my buddies encountered this question during a JavaScript job interview:

Can you explain how this counter functions? In other words, what does the non-minified version look like?

    let Counter = (_=1)=>$=>_++
    let c1 = Counter()
    console.log(c1()) //1
    console.log(c1()) //2

javascript

Answer 1

Answer №1

The following code snippet demonstrates the concept:

const IncrementCounter = function(value = 1) {
  return function increment() {
    return value++;
  };
}

When you execute IncrementCounter(), it initializes the counter with a default value of 1. Subsequently, as you call the returned count function, it retrieves the current value of the counter and increments it by 1.

Answer 2

The following code snippet demonstrates the concept:

const IncrementCounter = function(value = 1) {
  return function increment() {
    return value++;
  };
}

When you execute IncrementCounter(), it initializes the counter with a default value of 1. Subsequently, as you call the returned count function, it retrieves the current value of the counter and increments it by 1.

Answer 3

Answer №2

(_=1)=>$=>_++

This function utilizes arrow syntax and is equivalent to:

function f(_ = 1) {
    return $ => _++;
}

$ => _++; is also an arrow function that captures the closure _, increments it, and returns the value (postfix increment, so it returns the value first and then increments it):

function f(_ = 1) {
    return function($) { return _++; };
}

Therefore, the code translates to:

function Counter(count = 1) {
    return function() { return count++; };
}

(renamed _ to count and removed redundant $ variable)

The Counter function will return the previous value incremented by 1 each time, starting from the initial count value, similar to a counter.

Answer 4

(_=1)=>$=>_++

This function utilizes arrow syntax and is equivalent to:

function f(_ = 1) {
    return $ => _++;
}

$ => _++; is also an arrow function that captures the closure _, increments it, and returns the value (postfix increment, so it returns the value first and then increments it):

function f(_ = 1) {
    return function($) { return _++; };
}

Therefore, the code translates to:

function Counter(count = 1) {
    return function() { return count++; };
}

(renamed _ to count and removed redundant $ variable)

The Counter function will return the previous value incremented by 1 each time, starting from the initial count value, similar to a counter.

What makes (_=1)=>$=>_++ act as a tally?

Answer №1

Answer №2

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