Here's the code: this Display the table of any number given by user.

function table(num){

    var num = prompt("please enter any number");
    var x = num;

    if (num <= 0){
        alert("invalid number or Zero") ;
    } else {
        for(var i=1; i <= 10; i++){
            var y = x * i;

            document.write(x + ' X ' + i +' = ' + y + '<br/>')  ;
        }
    }
}

table();

now if -1 entered it display alert("invalid number or Zero"); and code breaks nothing is displayed. What I am looking for is how will it go again to start of function and prompt again for the number.

Here's the code: this Display the table of any number given by user.

function table(num){

    var num = prompt("please enter any number");
    var x = num;

    if (num <= 0){
        alert("invalid number or Zero") ;
    } else {
        for(var i=1; i <= 10; i++){
            var y = x * i;

            document.write(x + ' X ' + i +' = ' + y + '<br/>')  ;
        }
    }
}

table();

now if -1 entered it display alert("invalid number or Zero"); and code breaks nothing is displayed. What I am looking for is how will it go again to start of function and prompt again for the number.

• javascript
• function
• line
• goto

• 1 call table(0) again after the alert() ..see recursion – Amitd Commented Jun 6, 2015 at 9:04
• so i should call the method table(); inside the if statement again?? after alert. – Sam Commented Jun 6, 2015 at 9:07
• 1 what to you need the function parameter num for? Why do you copy it to x – PhilLab Commented Jun 6, 2015 at 9:07

4 Answers 4

Recursion is your friend: call the method again

function table(num){

    var num = prompt("please enter any number");
    var x = num;

    if (num <= 0){
        alert("invalid number or Zero") ;
        table(); // <----------------
    } else {
        for(var i=1; i <= 10; i++){
            var y = x * i;

            document.write(x + ' X ' + i +' = ' + y + '<br/>')  ;
        }
    }
}

Apart from that, you might want to consider a slight refactoring, since there are some useless variables/parameters

As has been mentioned before, you'll want to call the table function again (recursion), after the alert().

You can also clean up the function a bit, since there's no need to copy num to x, and a num parameter in the function isn't necessary, because you're initializing num in the function itself, and you have no need to pass it as a parameter:

function table(){
    var num = prompt("please enter any number");

    if (num <= 0){
        alert("invalid number or Zero") ;
        table();
    } else {
        for(var i = 1; i <= 10; i++){
            var y = num * i;
            document.write(num + ' X ' + i +' = ' + y + '<br/>');
        }
    }
}

table();

You could use a while loop as well.

Have a fiddle: https://jsfiddle/hmea6rLx/

JS code:

function table(num){
    var num=0;
    while (num <= 0){
            num = prompt("please enter any number");
            var x = num;
        $('#error').html("invalid number or Zero") ;
    }
    for(var i=1; i <= 10; i++){
        var y = x * i;

        $('#error').html(x + ' X ' + i +' = ' + y + '<br/>')  ;
    }
}

table();

Why the parameter (num)?

Your function rewritten in two ways, reusing itself:

document.querySelector('#table').addEventListener('click', start);
document.querySelector('#table2').addEventListener('click', start);


function start() {
  return window[this.id]();
}

function table(){
    var num = prompt("please enter any number");
    if (num <= 0){
        log(num + ": invalid number or zero");
        return table();
    } 
    for(var i = 1; i <= 10; i += 1){
      log(i + ' * ' + num + ' = ' + (i * num));
    }
}

// just for fun: more functional
function table2(){
    var num = prompt("please enter any number");
    var reslt = num <= 0 ? [num + ": invalid number or zero\n"] :
                [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].map(
                  function(v, i) {
                    return v + ' * ' + num + ' = ' + (v * num);
                  }
                );  
    log(reslt.join('\n'));
    return reslt.length < 10 ? table2() : true;
}

function log(str) {
  log.el = log.el || document.querySelector("#result");
  log.el.textContent += str+'\n';
}

<button id="table">exec table</button>
<button id="table2">exec table2</button>
<pre id="result"></pre>