Skip to content

Latest commit

 

History

History
146 lines (123 loc) · 3.15 KB

functions_js.md

File metadata and controls

146 lines (123 loc) · 3.15 KB
Raw

Functions

Basics

  • holds an algorithm, solution to a problems

  • called also logic

    • formal step-by-step description on how to do/achieve/solve something
    • imagine a recipe for cooking

  • frozen thought

  • set of instructions for the computer to achieve something

  • declaration:

    • name
    • input values (arguments)
    • return type

  • definition: declaration + body. In JS no separate declaration.

  • usage - call function. Apply recipe. Put arguments in parentheses after the name of the function. It returns your result.

function circleArea(radius) {
    return Math.PI * radius * radius; 
}

> circleArea(5)
78.53981633974483
  • arrow functions
    • if one line, the expression will be returned automatically. No return needed
    • if in block use {...}; if you want to return you need to use return
> const circleArea2 = radius => Math.PI * radius * radius
  • anonymous functions
> radius => Math.PI * radius * radius
[Function (anonymous)]
> (radius => Math.PI * radius * radius)(5)
78.53981633974483
  • typeof function
> typeof (radius => Math.PI * radius * radius)      // parenthesis because of eval. priority
'function'
> typeof circleArea
'function'
> typeof circleArea2
'function'
  • callback function
    • callback can be a parameter
    • which is applied by the code of the function
    • to perform some modification on the data processed in the function

Functional Programming

  • collections + functions
  • heavy use of callbacks

.forEach()

> let numbers = [3, 54, 4e1, 4]
> numbers
[ 3, 54, 40, 4 ]
> numbers.forEach((elem, index) => {
...     console.log(`${index} contains the value ${elem}`)
... })
0 contains the value 3
1 contains the value 54
2 contains the value 40
3 contains the value 4
> numbers.forEach((elem, index) => console.log(`${index} contains the value ${elem}`))
...
> numbers.forEach(number => console.log(number))
3
54
40
4
  • task: compute and print the circleArea for all numbers
> numbers.map(circleArea)
[
  28.274333882308138,
  9160.884177867836,
  5026.548245743669,
  50.26548245743669
]

.map()

  • .forEach() returns undefined. .map() returns the list of modified values. Q: how many elements will the result have?
  • task: double the integer of an array
> numbers.map(n => n*2)
[ 6, 108, 80, 8 ]
  • task: lowercase and remove leading spaces of an string array
> let names = ["   PETER", "   john", "Marry  "]
> names.map(name => name.trim().toLowerCase())
[ 'peter', 'john', 'marry' ]

.filter()

  • task: remove empty strings from an array
> let tokens = ["", "one", "two", "", "three"]
> tokens.filter(t => t.length > 0)
[ 'one', 'two', 'three' ]
> tokens.filter(t => t)
[ 'one', 'two', 'three' ]
  • task: return only even numbers
> numbers.filter(n => (n % 2) == 0)
[ 54, 40, 4 ]
> numbers.filter(n => !(n % 2))
[ 54, 40, 4 ]

.reduce() - reduce down to one value while aggregating

array.reduce(function(total, currentValue, currentIndex, arr), initialValue)

  • task: sum()
> numbers.reduce((acc, next) => acc + next, 0)
101
  • task: max()
> numbers.reduce((acc, next) => (next > acc ? next : acc), numbers[0])
54