What is a callback? A callback is a function to be executed after another function is executed. Sounds tongue-twisted? Normally if you want to call function do_b after function do_a the code looks something like
function do_a(){
console.log( '`do_a`: this comes out first');
}
function do_b(){
console.log( '`do_b`: this comes out later' );
}
do_a();
do_b();
Result
`do_a`: this comes out first `do_b`: this comes out later
However javascript is an event driven language. If do_a takes longer than do_b, the result of do_b comes out first than do_a;
function do_a(){
// simulate a time consuming function
setTimeout( function(){
console.log( '`do_a`: this takes longer than `do_b`' );
}, 1000 );
}
function do_b(){
console.log( '`do_b`: this is supposed to come out after `do_a` but it comes out before `do_a`' );
}
do_a();
do_b();
Result
`do_b`: this is supposed to come out after `do_a` but it comes out before `do_a` `do_a`: this takes longer than `do_b`
So how do we make sure do_b comes out after do_a in that situation? This is where callbacks comes in handy.
function do_a( callback ){
setTimeout( function(){
// simulate a time consuming function
console.log( '`do_a`: this takes longer than `do_b`' );
// if callback exist execute it
callback && callback();
}, 3000 );
}
function do_b(){
console.log( '`do_b`: now we can make sure `do_b` comes out after `do_a`' );
}
do_a( function(){
do_b();
});
Result
`do_a`: this takes longer than `do_b` `do_b`: now we can make sure `do_b` comes out after `do_a`
Different ways of applying a callback
function basic( callback ){
console.log( 'do something here' );
var result = 'i am the result of `do something` to be past to the callback';
// if callback exist execute it
callback && callback( result );
}
function callbacks_with_call( arg1, arg2, callback ){
console.log( 'do something here' );
var result1 = arg1.replace( 'argument', 'result' ),
result2 = arg2.replace( 'argument', 'result' );
this.data = 'i am some data that can be use for the callback function with `this` key word';
// if callback exist execute it
callback && callback.call( this, result1, result2 );
}
// this is similar to `callbacks_with_call`
// the only difference is we use `apply` instead of `call`
// so we need to pass arguments as an array
function callbacks_with_apply( arg1, arg2, callback ){
console.log( 'do something here' );
var result1 = arg1.replace( 'argument', 'result' ),
result2 = arg2.replace( 'argument', 'result' );
this.data = 'i am some data that can be use for the callback function with `this` key word';
// if callback exist execute it
callback && callback.apply( this, [ result1, result2 ]);
}
basic( function( result ){
console.log( 'this callback is going to print out the result from the function `basic`' );
console.log( result );
});
console.log( '--------------------------------------------------------------------------------------' );
( function(){
var arg1 = 'i am argument1',
arg2 = 'i am argument2';
callbacks_with_call( arg1, arg2, function( result1, result2 ){
console.log( 'this callback is going to print out the results from the function `callbacks_with_call`' );
console.log( 'result1: ' + result1 );
console.log( 'result2: ' + result2 );
console.log( 'data from `callbacks_with_call`: ' + this.data );
});
})();
console.log( '--------------------------------------------------------------------------------------' );
( function(){
var arg1 = 'i am argument1',
arg2 = 'i am argument2';
callbacks_with_apply( arg1, arg2, function( result1, result2 ){
console.log( 'this callback is going to print out the result from the function `callbacks_with_apply`' );
console.log( 'result1: ' + result1 );
console.log( 'result2: ' + result2 );
console.log( 'data from `callbacks_with_apply`: ' + this.data );
});
})();
Result
do something here this callback is going to print out the result from the function `basic` i am the result of `do something` to be past to the callback -------------------------------------------------------------------------------------- do something here this callback is going to print out the results from the function `callbacks_with_call` result1: i am result1 result2: i am result2 data from `callbacks_with_call`: i am some data that can be use for the callback function with `this` key word -------------------------------------------------------------------------------------- do something here this callback is going to print out the result from the function `callbacks_with_apply` result1: i am result1 result2: i am result2 data from `callbacks_with_apply`: i am some data that can be use for the callback function with `this` key word
Now we know how to execute node.js code, using require and exports, the importance of function scopes and closures and callbacks. Next we are going to take a look at the most important thing in node.js – Events.
