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7-callback-function.js
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7-callback-function.js
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// NodeJS often uses a callback pattern where if an error is encountered during execution, this error is passed
// as the first argument to the callback.
// DEFINING AN ERROR-FIRST CALLBACK
// There’s really only two rules for defining an error-first callback:
// 1. The first argument of the callback is reserved for an error object.
// If an error occurred, it will be returned by the first err argument.
// 2. The second argument of the callback is reserved for any successful
// response data. If no error occurred, err will be set to null and any successful
// data will be returned in the second argument.
// Example: error first callback function
function strictAddition(x, y, callback) {
if(typeof x !== 'number') { callback( new Error('First argument is not a number') ); return; }
if(typeof y !== 'number') { callback( new Error('Second argument is not a number') ); return; }
var result = x + y; setTimeout(function() { callback(null, result); }, 500);
}
function callback(err, data) { if(err) { console.log(err); return; } console.log(data);}
strictAddition(2, 10, callback);
strictAddition(-2, 10, callback);
strictAddition('uh oh', 10, callback);
strictAddition(2, '10', callback);
// MyExample:
function divide(x,y,callback)
{
if(y==0)
{
return callback(new Error("Division not possible "));
}
var d=x/y;
callback(null,d);
}
function res(err,result)
{
if(err)
{
return console.log(err);
}
else{
console.log("result of the divison is: "+result);
}
}
divide(35,5,res);