I create a simple function to display the names of 3 users on the page, but how can I not understand why I get undefined ? I pass an array of names to a function.

 function loadProfiles(userNames){ if (userNames.length > 3) { let loadingMessage = "This might take a while..."; console.log(loadingMessage); } else { let flashMessage = "Loading profiles"; console.log(flashMessage); } } console.log(loadProfiles(["user1","user2","user3","user4"]));
  • 3
    Where is return? Your function returns nothing - Alexey Ten
  • @ stas.t gives you the correct comment above. Why didn't you read the documentation on how to write javascript functions? - Vasily Barbashev
  • Give more complete code where and how flashMessage and loadingMessage - Kromster are used
  • The fact of the matter is that I do not use flashMessage and loadingMessage yet. the whole code consists of the example above and the index.html file which calls the loadProfiles function (["user1", "user2", "user3", "user4"]); and passes it an array of names. figured out like. - spectre_it

1 answer 1

In javascript, as in any other programming language, functions are performed directly and indirectly.

Straight lines, for example, add two numbers and return

 function sum(a, b) { return a + b; } // в этом случае результат роботы функции можно записывать в переменную var rez = sum(2,2); console.log(rez); // или просто выводить в лог console.log(sum(1,2));

Side effects of functions are those actions that they perform with the environment in which they are called, for example, the same addition, but the result is written to the global visibility variable, and is not returned as a result

 var rez; function sum(a, b) { rez = a + b; } // функция еще не отработала значение `undefined` console.log(rez); // функция уже отработала однако видим значение `undefined` потому что функция ничего нам не возвращает console.log(sum(1, 2)); // видим тройку потому что во время работы функции в предыдущей инструкции она совершила побочное действие над переменной `var rez` присвоив ей сумму передаваемых параметров console.log(rez);

It is worth noting that in javascript functions always return something, if the return instruction is not explicitly specified in the body, then the return value is undefined

These things can be combined, for example

 function sum(a, b) { console.log('я посчитала'); return a + b; } console.log(sum(1, 2));

Here the return of the sum of numbers is a direct action, and the output of the message to the log is side.

Now about your example. You have a function that performs only side effects, in this case passing an array of four names as parameters. You get into a block.

 if (userNames.length > 3) { let loadingMessage = "This might take a while..."; console.log(loadingMessage); }

And you actually get this message to the log, however, since you are accessing the function through console.log too, it displays undefined in the second line, since the function returns nothing.

Rewrite the example and you will get the expected (I hope :)) result

 function loadProfiles(userNames){ if (userNames.length > 3) { let loadingMessage = "This might take a while..."; console.log(loadingMessage); } else { let flashMessage = "Loading profiles"; console.log(flashMessage); } } loadProfiles(["user1","user2","user3","user4"]);

or so

 function loadProfiles(userNames) { if (userNames.length > 3) { let loadingMessage = "This might take a while..."; return loadingMessage; } else { let flashMessage = "Loading profiles"; return flashMessage; } } console.log(loadProfiles(["user1", "user2", "user3", "user4"]));
  • Thank you for the detailed response. how did you help me Thank you very much!!! - spectre_it