How to understand “add(1)(2)(3)=6”?

Question

I am learning curry function in javascript. And a question occurs to me.

// how to implement the add function in the below.

add(1)(2)(3) = 6;
add(1, 2, 3)(4) = 10;
add(1)(2)(3)(4)(5) = 15;

I've known the implemented code

function add() {
var _args = Array.prototype.slice.call(arguments);
var _adder = function () {
    _args.push(...arguments);
    return _adder;
};
_adder.toString = function () {
    return _args.reduce(function (a, b) {
        return a + b;
    });
}
return _adder;
}
console.log(add(1)(2)(3)(4)(5))    // function 
console.log(add(1)(2)(3))          // function
console.log(add(1, 2, 3)(4))       // function

console.log(add(1)(2)(3)(4)(5) == 15)  // true 
console.log(add(1)(2)(3) == 6)         // true
console.log(add(1, 2, 3)(4) == 10)     // true

console.log(add(1)(2)(3)(4)(5) === 15)  // false
console.log(add(1)(2)(3) === 6)         // false
console.log(add(1, 2, 3)(4) === 10)     // false

I know how the implemented code works. But I am very curious about the question. From my view, " add(1)(2)(3) = 6; " means that after executing the expression "add(1)(2)(3)", it should return a value which is fully equals to Number 6. But from this question and its implement codes, I may have a misunderstanding about the question. SO, what does the question REALLY mean? This question is often asked by the interviewer.

Answer 1

This comes down to the JavaScript operators.

= does assignment.

=== does a normal equality comparison. A function never equals a number.

== does troublesome type conversions on the two values, then an equality test. It's usually better to avoid it. In this case, those type conversions can convert your curried function to a string, which might call your toString method if its complicated rules make it coerce the first arg to a string, which runs your curried function, and it converts the other argument to a string, and the two strings are equal.

This kind of automatic-conversion-and-hope-for-the-best test is easy to do the wrong thing. Examples:

• console.log(add(10)(-10) == '') --> true
• console.log(add(10)(20) == 30) --> true
• console.log(add(10)(20) == ' 30 ') --> true
• console.log(add(10)(20) == 036) --> true
• console.log(add(10)(20) == ' 036 ') --> false
• console.log(add(10)(20) == ' 36 ') --> false
• console.log(add(2)(-2) == []) -> false
• console.log(0 == []) --> true

Also see https://stackoverflow.com/a/359509/1682419