Javascript function: no return statement identical to return undefined?

Question

var f1 = () => {
    return undefined;
};

var f2 = () => {
};

var a = f1(); // undefined
var b = f2(); // undefined

I know the results but I want to know more in-depth. Is that mean in Javascript functions, no return statement identical to return undefined?

Well, there's no return value. How would you call a return value that has not been defined? :) — Federico klez Culloca, Jun 27 '18 at 09:35
Every expression results in a value, otherwise it would be a statement. A function call is an expression. The value for "no value" is `undefined`. — deceze, Jun 27 '18 at 09:37
A statement is something like `for (...)`; you can only use it by itself, it does not result in a value. An expression is something that results in a value and can be used as part of a larger expression or statement. — deceze, Jun 27 '18 at 09:44

T.J. Crowder · Accepted Answer · 2018-06-27T09:56:35.710

I know the results but I want to know more in-depth. Is that mean in Javascript functions, no return statement identical to return undefined?

It is in effect, yes. The specification differentiates between the two, but in pragmatic terms, calling a function that "falls off the end" vs. return; vs. return undefined; all have exactly the same end result in terms of what have the call results in: undefined.

In my answer to the dupetarget (I should have realized!) I explain how the spec differentiates them, but again, it's just a spec distinction, not something you can observe in actual code.

score 3 · Answer 2 · answered Jun 27 '18 at 09:38

3

It depends.

For functions who are not used as an instance the default return value is undefined.

For a constructor, called with new, it returns this object as default.

Sources:

answered Jun 27 '18 at 09:38

Nina Scholz

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score 1 · Answer 3 · answered Jun 27 '18 at 09:39

If there is no return the return value will be undefined it's basically the same as doing return; (without value) since the function will "return" when it is finised (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/return for information to return)

If you initialize a function (more like a class) than the returned value will be the instance of this function instead.

Javascript function: no return statement identical to return undefined?

3 Answers3

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