JavaScript recursion detailed

1. What is recursion?

If a function can call itself internally, then this function is recursive. Simple understanding: a function calls itself internally, and this function is a recursive function.

As shown below:

function fn(){
 fn();
}
fn();

This function is a recursive function. When we print it directly, it will:

I found a printing error. Why? Because recursive functions have the same effect as loops. When there is no return value given to him, it will continue in an endless loop. So, we know:

Since recursion is prone to stack stack overflow errors, an exit condition return must be added.

How to write a correct recursive function? Take the above code as an example: for example, if we want to print 'hello' five times, we should write it like this:

var num = 1;
function fn(){
            console.log('Hello');
            if(num == 5){
                return;
            }
            num++;
            fn();
        }
        fn();

The print result is:

Now that we know what recursion is, let’s take a look at how to use recursion to solve problems!

2. Solve math problems using recursion

1. Find the factorial of 1 * 2 * 3 * 4 …*n

The code is as follows:

 function fn(n){
           if(n == 1){
               return 1;
           }
           return n*fn(n-1);
       }
       console.log('The factorial of 1-20 is: '+fn(20));
       console.log('The factorial of 1-10 is: '+fn(10));
       console.log('The factorial of 1-5 is: '+fn(5));

The print result is:

2. Find the Fibonacci sequence

The Fibonacci sequence, also known as the "rabbit sequence", refers to a sequence of numbers ：、1、1、2、3、5、8、13、21、34、……， that is, the value of the third term is the sum of the previous two terms. The user inputs an n and gets the number at that position.

The code is as follows:

function fb(n){
            if(n === 1 || n === 2){
                return 1;
            }
            return fb(n-1) + fb(n-2);
        }
       console.log('The value of the third Fibonacci number is: '+fb(3));
       console.log('The value of the 10th Fibonacci number is: '+fb(10));

The print result is:

3. Use recursion to find the corresponding data object

Return the corresponding data object according to id

There are objects like this:

        var date = [{
            id:1,
            name:'Electrical appliances',
            goods:[{
                id: 11,
                gname:'mobile phone'
            },{
                id: 12,
                gname: 'Computer'
            }]
        },{
            id:2,
            name:'clothing',
            goods:[{
                id : 21,
                gname:'pants'
            },{
                id : 22,
                gname : 'coat'
                }]
        },{
            id : 3,
            name: 'Food'
                }];

Now we need to return the corresponding data object by inputting id .

First, we can use for...Each() to traverse the array and get each value, as shown below:

function getId(array,id){
                    array.forEach(function(value){
                        console.log(value);
                    })
                }
                getId(date,1);

The printed result is:

At this time, if we want to get the value of the object with id 1, we can do this:

function getId(array,id){
                    array.forEach(function(value){
                       if(value.id === id){
                           console.log(value);
                       }
                    })
                }
                getId(date,1);

The print result is:

Yes, but what if we want to get the value of the object with id 11? Obviously, calling the function directly is not feasible, because we only get the value of the outermost object through for...Each, but the specific classification of the inner layer is not obtained. At this time, we can get the value of the inner object by recursively calling the getId(array,id) function.

The operation is as follows:

 function getId(array,id){
                    array.forEach(function(value){
                       if(value.id === id){
                           console.log(value);
                       }else if(value.goods && value.goods.length != 0){
                            getId(value.goods,id);
                       }

                    })
                }
                // getId(date,1);
                getId(date,11);

The printed result is:

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