Returning early from a function: null, explicit undefined, implicit undefined?

Question

var input = document.getElementById("input").value;

function sendRequest {
  if (Validate.isNumber(input)) {
    input.focus();
    return;
  }

  // or

  if (Validate.isNumber(input)) {
    input.focus();
    return null;
  }

  // or

  if (Validate.isNumber(input)) {
    return input.focus();
  }

  // start Ajax call
  var xhttp = new XMLHttpRequest();
  ...
}

Above 3 function have same result (if validation fail ends the function execution)... So what is the difference? Which one is better?

@OriDrori No it just send the value to server and save it in the database. — RaxPat, Sep 30 '18 at 18:20
I would return a promise from the ajax call, and Promise.reject() (with some to show why it was rejected) if the input is a number. In this way you can react to success and failures (server or check). — Ori Drori, Sep 30 '18 at 18:22
`input` appears to be a string in your code (the value of the input element). It does not have a `.focus()` method? — Bergi, Sep 30 '18 at 18:26
Why would you want to `return null;`? The usual `return;` results in `undefined`, which should be fine. Also, what does the function usually return if the validation doesn't fail? — Bergi, Sep 30 '18 at 18:27
@OriDrori Noo, I am not asking about returning a promise or not, it's just a DOM input field focus(), wondering what is the difference — RaxPat, Sep 30 '18 at 18:27
I know. If the return value means something - return a promise to cover all cases. If not, just return to terminate the function. — Ori Drori, Sep 30 '18 at 18:30
@Bergi if the input passed the validation, then just a simple ajax call and send it over to server (No response). — RaxPat, Sep 30 '18 at 18:30
@OriDrori So what is the difference in above 3 ways to terminate a function execution? — RaxPat, Sep 30 '18 at 18:32
You're not doing anything with the result of function, so it doesn't matter. Just return. — Ori Drori, Sep 30 '18 at 18:34
@RaxPat If you're not returning anything in the case of doing the ajax call, then you shouldn't return anything in the case of an error either. — Bergi, Sep 30 '18 at 18:37

Buggy · Accepted Answer · 2018-09-30T19:47:50.030

EsLint have a good rule: consistent-return

Require return statements to either always or never specify values. If any code paths in a function return a value explicitly but some code path do not return a value explicitly, it might be a typing mistake, especially in a large function

Airbnb JavaScript Style Guide

// No implicit return with side effects
function foo(callback) {
  const val = callback();
  if (val === true) {
    // Do something if callback returns true
  }
}

So what should we return for nothing?

In JavaScript, undefined means a variable has been declared but has not yet been assigned a value.

null is an assignment value. It can be assigned to a variable as a representation of no value.

null vs undefined

TLDR: return null.

PS also take a look at Null object pattern. If success path return Promise then negative path can return Promise.resolve(null) - if consumer of the function already check value inside Promise and doesn't care why it empty.

Returning early from a function: null, explicit undefined, implicit undefined?

1 Answers1