Javascript tail recursion instead of loop

Question

In one of Douglas Crockford speeches, He favours the use of tail recursion over loops. this code was presented,

function repeat(myFunc) {
  if (myFunc !== undefined) {
    return repeat(myFunc);
  }
}

I thought to define a myFunc but don't know if a static counter can retain its state during function calls or use a global counter. but being new to javascript I wanted to ask first. How can this be used in an example, say to count down from 10 to 0? Thanks.

function repeat(myFunc, cnt) { cnt = cnt || 0; if (myFunc !== undefined && cnt !== 10) { return repeat(myFunc, ++cnt); } } — Rajaprabhu Aravindasamy
– Rajaprabhu Aravindasamy, Commented Apr 10, 2016 at 20:17
i think your browser is stuck while doing this the function is never stopped — Muhammad Waqas
– Muhammad Waqas, Commented Apr 10, 2016 at 20:18
The role of myFunc is irrelavant in the code you have given, either it is undefined from the start, and the function doesn't get called recursively, or it is not, and it runs indefinitely. I don't think Crockford would have presented this code. Maybe you forgot brackets somewhere? I mean: myFunc has to be called at some point... — trincot
– trincot, Commented Apr 10, 2016 at 20:20

Nina Scholz · Accepted Answer · 2016-04-10 20:28:08Z

3

You need to call the function myFunc somewhere -- and evaluate the result for further call of repeat.

function repeat(myFunc) {
    if (myFunc()) {
        repeat(myFunc);
    }
}

var count = 10;

repeat(function () {
    document.write(count + '<br>');
    count--;
    return count >= 0;
});

edited Apr 10, 2016 at 20:28

answered Apr 10, 2016 at 20:23

Nina Scholz

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Comments

trincot · Accepted Answer · 2016-04-10 21:10:09Z

2

Here is a version that keeps state without global variable:

function repeat(myFunc, arg) {
    if ((arg = myFunc(arg)) !== undefined) {
        repeat(myFunc, arg);
    }
}

repeat(function (count) {
    document.write(count + ',');
    count--;
    if (count >= 0) return count;
}, 10);

answered Apr 10, 2016 at 21:10

trincot

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Comments

guest271314 · Accepted Answer · 2016-11-03 19:04:54Z

2

How can this be used in an example, say to count down from 10 to 0?

Pass a Number to repeat, call repeat with decremented number as parameter until variable parameter is equal to 0

function repeat(n) {
  console.log(n)
  if (n) {
    return repeat(--n);
  }
}

repeat(10)

edited Nov 3, 2016 at 19:04

answered Apr 10, 2016 at 20:21

guest271314

1

4 Comments

trincot Over a year ago

The name myFunc is a bit misleading in this case :)

guest271314 Over a year ago

@trincot Yes. Tried to change original js at OP at little as possible. myFunc could be any varible; updated variable name to n

Fred J. Over a year ago

@guest271314 your solution does not print zero at the end. not withstanding this, the reason for the question is to understand how to call a function and the number count down was 'just a use case. Poor one I must add.

guest271314 Over a year ago

@FredJ. "your solution does not print zero at the end." ? Logs 0 at console here. Yes, a function could be called to decrement number passed as parameter, though not necessary to achieve expected results. - operator provides same results as calling a function to do the same.

Oriol · Accepted Answer · 2016-04-10 20:19:09Z

0

Not sure if I understand what approach you want, but you can use this to count down recursively

function repeat(myFunc, times) {
  if(times > 0 && typeof myFunc == 'function') {
    myFunc(times);
    repeat(myFunc, times-1);
  }
}
repeat(alert, 10);

answered Apr 10, 2016 at 20:19

Oriol

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