Function Arguments

Overview


Arguments are the inputs to a function. They are declared in the function definition 


function multiply(a, b){
  return a*b;
}

let result = multiply(5,10);


An error will not occur if the user enters in less arguments than declared in the function declaration. Any arguments not passed in the function call are given a special value called "undefined". 


function multiply(a, b){
  if(b===undefined) b= 10;
  return a*b;
}

//this is legal
let result = multiply(5);


Default Arguments

Default values can be assigned to function arguments in the function declaration. The following will assign b to be equal to 10 if it is not provided in the function call. 


function multiply(a, b=10){
	return a*b;
}
            	



let result1 = multiply(5,2);
//result1 will be equal to 10

let result2 = multiply(5);
//result2 will be equal to 50
Try it!

Variable Number of Arguments




function multiply(){
	let result = 1;
	for(let arg of arguments) result = result * arg;
	return result;
}

let result = multiply(2,2,3,4,5);
Try it!

Functions as Arguments to Functions

Specify the Name and Input dataset
Functions can be used like any other piece of data or variable. For example, a function can be passed to another function, as a parameter. This may seem like a fairly obscure fact that is not that useful, but it turns out to be incredibly useful in data querying and transformations. 


let data = [1,2,3,4,5];
let answer = [];

for(let number of data){
  if(number>3) answer.push(number);					
}
Try it!

This code effectively filters the list of numbers for numbers that are greater than 3.

Lets imagine that we put the filter condition in a function as follows. 


let data = [1,2,3,4,5];
let answer = [];
let condition = function(number){ 
  if(number>3) return true;
  else return false;
}

for(let number of data){
  if(condition(number)) answer.push(number);					
}
Try it! 

let data = [1,2,3,4,5];
let answer = [];
let condition = function(number){ 
  if(number>3) return true;
  else return false;
}

let filter = function(data, condition){
  let answer = [];
  for(let item of data){
    if(condition(item)) answer.push(item);					
  }
  return answer;
}

let filtered = filter(data, condition);
Try it!

Once we have a filter function defined as above, we can now filter an array very simply. 


let filtered = filter([1,2,3,4,5], p=>p>3);
Try it!

As an additional simplification, the filter function is defined on the array object, so that we can do the following. 


let filtered = [1,2,3,4,5].filter(p=>p>3);
Try it!

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