praticalFunction.md

JavaScript

Pratical Function

Age	Return or not	Return value
For	no	X
ForEach	no	X
Map	return array	Return all values by processing in the array
Filter	return array	Only return qualified values in the array
Find	return array	Only return the first value that meets the condition in the array
join	return string	Return a string with specific symbol

For

for (let i=0; i<items.length; i++) {
  copy.push(items[i])
}

ForEach

• Just like for, ForEach doesn't return value

var arr = [1,2,3,4]
var result =[ ]
var result = arr.forEach(
  function(item) {
    return item. 
  }
) 
console.log(result) //undefined，beacause ForEach doesn't return value

//coding Style 1 
var result =[ ]
arr.forEach(
  function(item) {
    result.push(item)
}) 

//coding Style 2
var result =[ ]
arr.forEach( item => result.push(item)) 

Map()

• Map only return array
• Map can get item&index
• It's available to custom data structure we need

var arr = [1,2,3,4]
var result = arr.map(item=> item*2 ); 
// [2,4,6,8]
var result2 = arr.map((item, index)=> {
  return { name: item, order: index+1 } 
}) 
// [{name:1 , order:1},{name:2 , order:2},{name:3,order:3},{name:4 ,order:4}]

• if using {} , there isreturn
• if useing (), there is no return

Filter()

• Only return qualified values in the array

var ele = [1,2,3,4]
var result = arr.filter( item=> item > 2); // [3,4]

Find()

• Only return the first value that meets the condition in the array

arr = [2,3,4,5]
found = arr.find( item => item > 2)
console.log(found);  // 3

join()

• return string with specific symbol

arr = [2,3,4,5]
newarr = arr.join(',') // 2,3,4,5

for of , for in

• for of: get object value
• for in: get object property

let list = [3,4,5]

for (let i in list){
  console.log(i)  // 0,1,2
}

for (let i of list){
  console.log(i)  // 3,4,5
}

最常搞混的函式 Slice, Splice,Split

• slice：把原陣列的部分複製下來（撕下來）

const arr = ['Jan', 'March', 'April', 'June'];
arr.slice(2) // 從2開始撕下來 ['April', 'June']
arr.slice(1,3) // 把2~3的元素撕下來['March', 'April']

• splice：把原陣列刪除/新增

const months = ['Jan', 'March', 'April', 'June'];
months.splice(1, 0, 'Feb'); // 從1開始，刪除0的元素，把'Feb'丟進去

console.log(months);
// expected output: Array ["Jan", "Feb", "March", "April", "June"]

months.splice(4, 1, 'May');  // 從4開始，刪除1個元素，把'May'丟進去

console.log(months);
// expected output: Array ["Jan", "Feb", "March", "April", "May"]

• split: 把字串切割

const str = 'The quick brown fox jumps over the lazy dog.';
const words = str.split(' ');
console.log(words[3]); // fox