javascript promise then run before resolve code example
Example 1: promise js
We use promise to make a AsyncFunction, cose simetime we have to wait that function give
us some result.
Example, if we use ajax we have await ajax data or statament.
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Make a simple example.
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var asyncronus_function= (number)=>
{
return new Promise( (accept, reject)=>
{
})
}
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this function return a promise Object, thet required a function executor
this functions (accept, reject) are defined in the executor
function, that was needed in promise constructor.
Syntax: new Promise (executor)
executor= (accept, reject) =>{}
if function end well we return a accept(), otherwise reject()
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Let complete asyncronus_function
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var asyncronus_function= (number)=>
{
return new Promise( (accept, reject)=>
{
if(number>10)
return accept("my first async");
return reject("my first async error")
})
}
if it dont return any of this 2 function, Promise state is [PENDING] ,
if return accept is [RESOLVED] end if return reject is [REJECTED]
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how we can retrieve accept or reject?
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there is two methods really important, that we have to consider afther we call this function
1) .then(function(error){}) is call when promise state is [RESOLVED]
2) .error(function(error){}) is call when promise state is [REJECTED]
3) do nothing if [PENDING]
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let call asyncronus_function()!!!
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asyncronus_function(MY_NUMBER).then(function(data)
{
console.log(data)
}).catch(error =>
{
console.log(error)
});
if MY_NUMBER> 10 ,asyncronus_function print data : OUTPUT my first async
if MY_NUMBER<10 , asyncronus_function print error : OUTPUT my first async error
HOPE it halp and have a nice day!
Example 2: promise chaining
new Promise(function(resolve, reject) {
setTimeout(() => resolve(1), 1000);
}).then(function(result) {
alert(result);
return result * 2;
}).then(function(result) {
alert(result);
return result * 2;
}).then(function(result) {
alert(result);
return result * 2;
});
Example 3: .then javascript
const promise2 = doSomething().then(successCallback, failureCallback);