js currying function example

问题

How to understand the currying function?

How the newSum and newFind works?

var currying = function(fn) {
  var args = [];
  return function() {
    if (!!arguments.length) {
      [].push.apply(args, arguments); // What's the meaning of this writing?
      return arguments.callee;
    } else {
      return fn.apply(this, args);
    }
  }
}

// accumulation currying
var sum = (function(num){
  var ret = 0;
  return function(){
    for(var i = 0, len = arguments.length; i < len; i++) {
        ret += arguments[i];
    }
    return ret;
  }
})();
var newSum = currying(sum);
newSum(1)(2)(3)(4)()  // 10

// element find currying
var find = function(arr, el){
  return arr.indexOf(el) !== -1;
}

var newFind = currying(find)([1,2,3]);
newFind(1);
newFind(2);

回答1:

The currying function, and gets a function as an argument, and returns a new function, that when invoked:

If arguments are provided, they are accumulated in the args array
If arguments are not provided, the original function is called with all accumulated arguments.

So, if we look at this call for example: newSum(1)(2)(3)(4)() - there are 5 function invocations:

Calling newSum with 1. Getting the curried function with 1 accumulated.
Calling the curried function with 2 - getting the same function with 1 and 2 accumulated, and so on for 3 and 4.
Calling the curried function without arguments - applying all the accumulated arguments (1, 2, 3, 4) to the original sum function - and getting the correct value, 10.

About [].push.apply(args, arguments); and fn.apply(this, args);: apply is a method on Function.prototype that basically lets you call a function, supplying a context object and an array of arguments. So basically [].push.apply(...) is a trick of concatenating an array into another array, in our case, concat arguments (which is the list of arguments provided to a function upon invocation) into args (the accumulating list of arguments). fn.apply(this, args); is simply calling the original function with all the accumulated arguments. You can read more about it in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply

By the way, newFind(1); and newFind(2); both return a function, that will look for the index of the element only when invoked, meaning newFind(1)() === true

来源：https://stackoverflow.com/questions/46745206/js-currying-function-example

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javascript

currying