Functions are first-class citizen

Functions are first-class citizen in JavaScript, as the same as other types(e.g. number, array, object). They can be used as arguments, as well as return value from other function.

Take a simple example, if we aim to print out all the elements in an array, most people probably will do it this way:

function printWithLoop(arr) {
    for (let i = 0, len = arr.length; i < len; i++) {
        console.log(arr[i]);
    }
}

If you're a bit familar with higher-order function, you may be inclined to use Array.prototype.forEach:

function printWithIterator(arr) {
    (arr || []).forEach(it => {
        console.log(it);
    });
}

We can then simplify the code further with：

function simplePrint(arr) {
    (arr || []).forEach(console.log);
}

Have a second thought here, is the output from simplePrint exactly the same as printWithIterator? If not, can you explain what makes the difference?

Function overloading

Function overloading gives the ability for functions to behave differently based on the number or type of the arugments. E.g. Array.from.

• When given a single argument, it simplely creates a new Array instance from an array-like or iterable object

let set = new Set(['foo', 'bar', 'foo']);
console.log(Array.from(set)); //["foo", "bar"]

• When given a second argument mapFn which is optioinal, it will apply mapFn to each element when creating the new array.

console.log(Array.from([1, 2, 3], x => x + x)); // [2, 4, 6] 

Curry

Curry, also called partial application. Currying a function basically means that a function will absord some arguments and return another function for later invocation. The returning function can have access to the already absorded arguments through closure.

Parameters vs Arugments

First we need to understand two basic concepts in functions.

• Parameters: the placeholders in function declarations. We can use function.length to get the number of parameters.

function A(a, b) {}
// a and b are parameters
console.log(A.length); //2

• Arguments: the values passed to functions when functions are applied. We can use arguments to get the list of arguments.

function B(a, b) {
    console.log(arguments);
}

B(1,2,3); // 1,2,3

To conclude, parameters are what you expect while arguments are what you got.

Curry example

Assume we have a function to compute the sum of three numbers.

function sum(x, y, z) {
    console.log(x + y + z);
}

sum(1,2,3); //6

If we want to achieve the following result:

sum(1,2,3); //6
sum(1)(2,3); //6
sum(1,2)(3); //6

Have a deep look, what we want to achieve is that when the function sum receives the arguments it expects (i.e. three numbers), it will compute their sum and return the value. Otherwise, it will keep returning another function (e.g. sum(1) and sum(1,2) both return another function) which can be invoked with more numbers. This is Curry!

function curry(fn) { //Let's ignore the function context for simplicity
    return function f(...args) {
        /**
         * if the number of passed in arguments is more than what we expect
         * invoke the function and return the result
         */
        if(args.length >= fn.length) { 
            return fn.apply(this, args);
        } else {
            //return another function which can access to the passed arguments through closure
            return function(...arr) { 
                return f.apply(this, args.concat(arr));
            }
        }
    }
}

let sumWithCurry = curry(sum);
sumWithCurry(1,2,3); //6
sumWithCurry(1)(2,3); //6
sumWithCurry(1,2)(3); //6

Function.prototype.bind

Function.prototype.bind has two main functionalities:

• binding the function context this
• curry

function sayHi(greeting, ending) {
    console.log(`My name is ${this.name}, ${greeting}. ${ending}!`);
}

let fn = sayHi.bind({name: 'mike'}, 'Love you'); // greeting is absorded
fn('Thanks!'); // My name is mike, Love you. Thanks!!

In development, we can use curry to write more elegant code:

function print(arr) {
    console.log(arr.join('|'));
}

let arr = [1,2,3];
setTimeout(function(){
    print([1,2,3]);
}, 1000);
// 1|2|3

is equivalent to

function print(arr) {
    console.log(arr.join('|'));
}

let arr = [1,2,3];
setTimeout(print.bind(null, arr), 1000);
// 1|2|3

Reference

• Effective JavaScript

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