javascript 递归函数调用(recursive funciton call)
所谓的递归函数调用,就是自己调用自己的函数。
1 var timerHandler = null; 2 function a(){ 3 console.log(123); 4 timerHandler = setTimeout(a, 1000) ; 5 } 6 a(); 7 //clearTimeout(timerHandler);
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/* Count down to 0 recursively. */ var functionHolder = function (counter) { console.log(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 output3
2
1
0
as above. If I then changedfunctionHolder
as follows:
functionHolder = function(whatever) { console.log("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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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).