js currying function example

Question

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); 

Answer 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