when should callback functions in JavaScript be used
can someone explain me how should we know that when we should use a callback?
like in the code given here as a link
snip of code is given here
we see that in readFile method inside fetchAll(cb), we used callback denoted by (cb) to read the content, parse it and stringify it and whatever, but in readFile method of save(), there was no need to use (cb). So how can we know when to use the callback?
node.js
edited Nov 23 '18 at 14:36
Luke Walker
27314
asked Nov 23 '18 at 12:53
Ratnabh kumar raiRatnabh kumar rai
11
add a comment |
can someone explain me how should we know that when we should use a callback?
like in the code given here as a link
snip of code is given here
we see that in readFile method inside fetchAll(cb), we used callback denoted by (cb) to read the content, parse it and stringify it and whatever, but in readFile method of save(), there was no need to use (cb). So how can we know when to use the callback?
node.js
edited Nov 23 '18 at 14:36
Luke Walker
27314
asked Nov 23 '18 at 12:53
Ratnabh kumar raiRatnabh kumar rai
11
2
All relevant code need to be posted here directly as text. And you use a callback when you have code to run after since action, and you aren't sure when the action will complete.
– Carcigenicate
Nov 23 '18 at 12:58
add a comment |
can someone explain me how should we know that when we should use a callback?
like in the code given here as a link
snip of code is given here
we see that in readFile method inside fetchAll(cb), we used callback denoted by (cb) to read the content, parse it and stringify it and whatever, but in readFile method of save(), there was no need to use (cb). So how can we know when to use the callback?
node.js
edited Nov 23 '18 at 14:36
Luke Walker
27314
asked Nov 23 '18 at 12:53
Ratnabh kumar raiRatnabh kumar rai
11
can someone explain me how should we know that when we should use a callback?
like in the code given here as a link
snip of code is given here
we see that in readFile method inside fetchAll(cb), we used callback denoted by (cb) to read the content, parse it and stringify it and whatever, but in readFile method of save(), there was no need to use (cb). So how can we know when to use the callback?
node.js
node.js
edited Nov 23 '18 at 14:36
Luke Walker
27314
asked Nov 23 '18 at 12:53
Ratnabh kumar raiRatnabh kumar rai
11
edited Nov 23 '18 at 14:36
Luke Walker
27314
asked Nov 23 '18 at 12:53
Ratnabh kumar raiRatnabh kumar rai
11
edited Nov 23 '18 at 14:36
Luke Walker
27314
edited Nov 23 '18 at 14:36
Luke Walker
27314
edited Nov 23 '18 at 14:36
Luke Walker
27314
27314
asked Nov 23 '18 at 12:53
Ratnabh kumar raiRatnabh kumar rai
11
asked Nov 23 '18 at 12:53
Ratnabh kumar raiRatnabh kumar rai
11
asked Nov 23 '18 at 12:53
Ratnabh kumar raiRatnabh kumar rai
11
11
2
All relevant code need to be posted here directly as text. And you use a callback when you have code to run after since action, and you aren't sure when the action will complete.
– Carcigenicate
Nov 23 '18 at 12:58
add a comment |
2
All relevant code need to be posted here directly as text. And you use a callback when you have code to run after since action, and you aren't sure when the action will complete.
– Carcigenicate
Nov 23 '18 at 12:58
2
2
All relevant code need to be posted here directly as text. And you use a callback when you have code to run after since action, and you aren't sure when the action will complete.
– Carcigenicate
Nov 23 '18 at 12:58
All relevant code need to be posted here directly as text. And you use a callback when you have code to run after since action, and you aren't sure when the action will complete.
– Carcigenicate
Nov 23 '18 at 12:58
add a comment |
2 Answers
2
active
oldest
votes
Its simple. just be aware what is the nature of the methods you are using.
readFile is async so it will need a callback. What it is basically saying is "hey, I am going to read the file you asked, but while I read it you can do other stuff instead of waiting for me and when I am finished, I will come back to you". In your readFile method in save() you still pass a callback:
(err, fileContent) => {
// do stuff
}
Callback are used to handle async code so we can continue work, while something else is happening and we do not want to stop and wait for it.
answered Nov 23 '18 at 14:37
squeekyDavesqueekyDave
383114
but in my readFIle method in fetchAll() also i passed callback like (err, fileContent) so why i used there cb ? what was its need? cant it be done without cb as it is done inside save()?
– Ratnabh kumar rai
Nov 23 '18 at 21:39
Post all relevant code, posting an image as external source is a bad practice. Post the code where you call fetchAll, because you are passing a function whwn you call it and I cannot see any of that in the picture.
– squeekyDave
Nov 23 '18 at 21:55
add a comment |
const fs=require('fs')
const path=require('path')
module.exports=class Prroduct{
constructor(title,imgurl,description,price){
this.title=title
this.imgurl=imgurl
this.description=description
this.price=price
}
save(){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
let products=
if(!err){
products=JSON.parse(fileContent)
}
products.push(this)
fs.writeFile(p,JSON.stringify(products),(err)=>{
console.log(err)
})
})
}
static fetchAll(cb){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
if(err){
cb()
}
cb(JSON.parse(fileContent))
})
}
}
answered Nov 25 '18 at 6:53
Ratnabh kumar raiRatnabh kumar rai
11
@squeekyDave here is the code
– Ratnabh kumar rai
Nov 25 '18 at 6:55
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Its simple. just be aware what is the nature of the methods you are using.
readFile is async so it will need a callback. What it is basically saying is "hey, I am going to read the file you asked, but while I read it you can do other stuff instead of waiting for me and when I am finished, I will come back to you". In your readFile method in save() you still pass a callback:
(err, fileContent) => {
// do stuff
}
Callback are used to handle async code so we can continue work, while something else is happening and we do not want to stop and wait for it.
answered Nov 23 '18 at 14:37
squeekyDavesqueekyDave
383114
but in my readFIle method in fetchAll() also i passed callback like (err, fileContent) so why i used there cb ? what was its need? cant it be done without cb as it is done inside save()?
– Ratnabh kumar rai
Nov 23 '18 at 21:39
Post all relevant code, posting an image as external source is a bad practice. Post the code where you call fetchAll, because you are passing a function whwn you call it and I cannot see any of that in the picture.
– squeekyDave
Nov 23 '18 at 21:55
add a comment |
Its simple. just be aware what is the nature of the methods you are using.
readFile is async so it will need a callback. What it is basically saying is "hey, I am going to read the file you asked, but while I read it you can do other stuff instead of waiting for me and when I am finished, I will come back to you". In your readFile method in save() you still pass a callback:
(err, fileContent) => {
// do stuff
}
Callback are used to handle async code so we can continue work, while something else is happening and we do not want to stop and wait for it.
answered Nov 23 '18 at 14:37
squeekyDavesqueekyDave
383114
but in my readFIle method in fetchAll() also i passed callback like (err, fileContent) so why i used there cb ? what was its need? cant it be done without cb as it is done inside save()?
– Ratnabh kumar rai
Nov 23 '18 at 21:39
Post all relevant code, posting an image as external source is a bad practice. Post the code where you call fetchAll, because you are passing a function whwn you call it and I cannot see any of that in the picture.
– squeekyDave
Nov 23 '18 at 21:55
add a comment |
Its simple. just be aware what is the nature of the methods you are using.
readFile is async so it will need a callback. What it is basically saying is "hey, I am going to read the file you asked, but while I read it you can do other stuff instead of waiting for me and when I am finished, I will come back to you". In your readFile method in save() you still pass a callback:
(err, fileContent) => {
// do stuff
}
Callback are used to handle async code so we can continue work, while something else is happening and we do not want to stop and wait for it.
answered Nov 23 '18 at 14:37
squeekyDavesqueekyDave
383114
Its simple. just be aware what is the nature of the methods you are using.
readFile is async so it will need a callback. What it is basically saying is "hey, I am going to read the file you asked, but while I read it you can do other stuff instead of waiting for me and when I am finished, I will come back to you". In your readFile method in save() you still pass a callback:
(err, fileContent) => {
// do stuff
}
Callback are used to handle async code so we can continue work, while something else is happening and we do not want to stop and wait for it.
answered Nov 23 '18 at 14:37
squeekyDavesqueekyDave
383114
answered Nov 23 '18 at 14:37
squeekyDavesqueekyDave
383114
answered Nov 23 '18 at 14:37
squeekyDavesqueekyDave
383114
answered Nov 23 '18 at 14:37
squeekyDavesqueekyDave
383114
383114
but in my readFIle method in fetchAll() also i passed callback like (err, fileContent) so why i used there cb ? what was its need? cant it be done without cb as it is done inside save()?
– Ratnabh kumar rai
Nov 23 '18 at 21:39
Post all relevant code, posting an image as external source is a bad practice. Post the code where you call fetchAll, because you are passing a function whwn you call it and I cannot see any of that in the picture.
– squeekyDave
Nov 23 '18 at 21:55
add a comment |
but in my readFIle method in fetchAll() also i passed callback like (err, fileContent) so why i used there cb ? what was its need? cant it be done without cb as it is done inside save()?
– Ratnabh kumar rai
Nov 23 '18 at 21:39
Post all relevant code, posting an image as external source is a bad practice. Post the code where you call fetchAll, because you are passing a function whwn you call it and I cannot see any of that in the picture.
– squeekyDave
Nov 23 '18 at 21:55
but in my readFIle method in fetchAll() also i passed callback like (err, fileContent) so why i used there cb ? what was its need? cant it be done without cb as it is done inside save()?
– Ratnabh kumar rai
Nov 23 '18 at 21:39
but in my readFIle method in fetchAll() also i passed callback like (err, fileContent) so why i used there cb ? what was its need? cant it be done without cb as it is done inside save()?
– Ratnabh kumar rai
Nov 23 '18 at 21:39
Post all relevant code, posting an image as external source is a bad practice. Post the code where you call fetchAll, because you are passing a function whwn you call it and I cannot see any of that in the picture.
– squeekyDave
Nov 23 '18 at 21:55
Post all relevant code, posting an image as external source is a bad practice. Post the code where you call fetchAll, because you are passing a function whwn you call it and I cannot see any of that in the picture.
– squeekyDave
Nov 23 '18 at 21:55
add a comment |
const fs=require('fs')
const path=require('path')
module.exports=class Prroduct{
constructor(title,imgurl,description,price){
this.title=title
this.imgurl=imgurl
this.description=description
this.price=price
}
save(){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
let products=
if(!err){
products=JSON.parse(fileContent)
}
products.push(this)
fs.writeFile(p,JSON.stringify(products),(err)=>{
console.log(err)
})
})
}
static fetchAll(cb){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
if(err){
cb()
}
cb(JSON.parse(fileContent))
})
}
}
answered Nov 25 '18 at 6:53
Ratnabh kumar raiRatnabh kumar rai
11
@squeekyDave here is the code
– Ratnabh kumar rai
Nov 25 '18 at 6:55
add a comment |
const fs=require('fs')
const path=require('path')
module.exports=class Prroduct{
constructor(title,imgurl,description,price){
this.title=title
this.imgurl=imgurl
this.description=description
this.price=price
}
save(){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
let products=
if(!err){
products=JSON.parse(fileContent)
}
products.push(this)
fs.writeFile(p,JSON.stringify(products),(err)=>{
console.log(err)
})
})
}
static fetchAll(cb){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
if(err){
cb()
}
cb(JSON.parse(fileContent))
})
}
}
answered Nov 25 '18 at 6:53
Ratnabh kumar raiRatnabh kumar rai
11
@squeekyDave here is the code
– Ratnabh kumar rai
Nov 25 '18 at 6:55
add a comment |
const fs=require('fs')
const path=require('path')
module.exports=class Prroduct{
constructor(title,imgurl,description,price){
this.title=title
this.imgurl=imgurl
this.description=description
this.price=price
}
save(){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
let products=
if(!err){
products=JSON.parse(fileContent)
}
products.push(this)
fs.writeFile(p,JSON.stringify(products),(err)=>{
console.log(err)
})
})
}
static fetchAll(cb){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
if(err){
cb()
}
cb(JSON.parse(fileContent))
})
}
}
answered Nov 25 '18 at 6:53
Ratnabh kumar raiRatnabh kumar rai
11
const fs=require('fs')
const path=require('path')
module.exports=class Prroduct{
constructor(title,imgurl,description,price){
this.title=title
this.imgurl=imgurl
this.description=description
this.price=price
}
save(){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
let products=
if(!err){
products=JSON.parse(fileContent)
}
products.push(this)
fs.writeFile(p,JSON.stringify(products),(err)=>{
console.log(err)
})
})
}
static fetchAll(cb){
const p=path.join(__dirname,'../','data','products.json')
fs.readFile(p,(err,fileContent)=>{
if(err){
cb()
}
cb(JSON.parse(fileContent))
})
}
}
answered Nov 25 '18 at 6:53
Ratnabh kumar raiRatnabh kumar rai
11
answered Nov 25 '18 at 6:53
Ratnabh kumar raiRatnabh kumar rai
11
answered Nov 25 '18 at 6:53
Ratnabh kumar raiRatnabh kumar rai
11
answered Nov 25 '18 at 6:53
Ratnabh kumar raiRatnabh kumar rai
11
11
@squeekyDave here is the code
– Ratnabh kumar rai
Nov 25 '18 at 6:55
add a comment |
@squeekyDave here is the code
– Ratnabh kumar rai
Nov 25 '18 at 6:55
@squeekyDave here is the code
– Ratnabh kumar rai
Nov 25 '18 at 6:55
@squeekyDave here is the code
– Ratnabh kumar rai
Nov 25 '18 at 6:55
add a comment |
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2
All relevant code need to be posted here directly as text. And you use a callback when you have code to run after since action, and you aren't sure when the action will complete.
– Carcigenicate
Nov 23 '18 at 12:58