Is there a way to check if a function returns ANY value at all. So for example:
if(loop(value) --returns a value--) {
//do something
}
function loop(param) {
if (param == 'string') {
return 'anything';
}
}
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4 Answers
Functions that don't return an object or primitive type return undefined. Check for undefined:
if(typeof loop(param) === 'undefined') {
//do error stuff
}
3 Comments
seamus
for other viewers do if( (typeof loop(param) ) === 'undefined') { //do error stuff } extra () important
José Cabo
if ( loop(param) === undefined ) {} // Works like in the answer and is "better".
tonix
How can I discriminate between a function which didn't return anything (e.g.:
function a() {} var ret = a(); // typeof ret === "undefined") and a function which explicitly returned undefined (e.g.: function a() { return void 0; // Returns undefined explicitly } var ret = a(); // typeof ret === "undefined")? Is there a way to know (from the caller side) that a function has returned through a return statement instead of implicitly? Thank youA function with no return will return undefined. You can check for that.
However, a return undefined in the function body will also return undefined (obviously).
1 Comment
tonix
How to differentiate the two cases (implicit
undefined return and explicit return undefined) from the caller side?You can do this :
if(loop(param) === undefined){}
That will work everytime with one exception, if your function return undefined, it will enter in the loop. I mean, it return something but it is undefined...
Comments
If you want to do something with the output of your function in the case it returns something, first pass it to a variable and then check if the type of that variable is "undefined".
Demo
testme("I'm a string");
testme(5);
function testme(value) {
var result = loop(value);
if(typeof result !== "undefined") {
console.log("\"" + value + "\" --> \"" + result + "\"");
} else {
console.warn(value + " --> " + value + " is not a string");
}
function loop(param) {
if (typeof param === "string") {
return "anything";
}
}
}
