Question

111

Calling a javascript function recursively

rated 0 times [ 116] [ 5] / answers: 1 / hits: 153447 / 13 Years ago, mon, august 15, 2011, 12:00:00

I can create a recursive function in a variable like so:

/* Count down to 0 recursively.

 */

var functionHolder = function (counter) {

    output(counter);

    if (counter > 0) {

        functionHolder(counter-1);

    }

}

With this, functionHolder(3); would output 3 2 1 0. Let's say I did the following:

var copyFunction = functionHolder;

copyFunction(3); would output 3 2 1 0 as above. If I then changed functionHolder as follows:

functionHolder = function(whatever) {

    output(Stop counting!);

Then functionHolder(3); would give Stop counting!, as expected.

copyFunction(3); now gives 3 Stop counting! as it refers to functionHolder, not the function (which it itself points to). This could be desirable in some circumstances, but is there a way to write the function so that it calls itself rather than the variable that holds it?

That is, is it possible to change only the line functionHolder(counter-1); so that going through all these steps still gives 3 2 1 0 when we call copyFunction(3);? I tried this(counter-1); but that gives me the error this is not a function.

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eastonb

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3 Years ago

answered 13 Years ago kourtney · Accepted Answer

Using Named Function Expressions:

You can give a function expression a name that is actually private and is only visible from inside of the function ifself:

var factorial = function myself (n) {

    if (n <= 1) {

        return 1;

    }

    return n * myself(n-1);

}

typeof myself === 'undefined'

Here myself is visible only inside of the function itself.

You can use this private name to call the function recursively.

See 13. Function Definition of the ECMAScript 5 spec:

The Identifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the Identifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

Please note that Internet Explorer up to version 8 doesn't behave correctly as the name is actually visible in the enclosing variable environment, and it references a duplicate of the actual function (see patrick dw's comment below).

Using arguments.callee:

Alternatively you could use arguments.callee to refer to the current function:

var factorial = function (n) {

    if (n <= 1) {

        return 1;

    }

    return n * arguments.callee(n-1);

}

The 5th edition of ECMAScript forbids use of arguments.callee() in strict mode, however:

(From MDN): In normal code arguments.callee refers to the enclosing function. This use case is weak: simply name the enclosing function! Moreover, arguments.callee substantially hinders optimizations like inlining functions, because it must be made possible to provide a reference to the un-inlined function if arguments.callee is accessed. arguments.callee for strict mode functions is a non-deletable property which throws when set or retrieved.