understanding javascript typeof

Question

This is a very basic issue i have as i am new to javascript but i cannot move forward until i understand this little thing.

i have this function..

function stringOrNot() {
  var input = prompt('Input something here');

  if (typeof input != 'string') {
    alert("That is not a string!");

  } else {
    alert("That is a string!");
  }
}

stringOrNot();

Question
What do i need to input to get the alert "That is not a string"?
Which kind of inputs are not strings?

Whatever i input i get 'That is a string!' returned.

also this function...

function stringOrNot() {
  var input = prompt('Input something here');

  if (typeof input != 'string') {
    alert("That is not a string!");
  } else (typeof input == 'string'){
    alert("That is a string!");
  }
}

stringOrNot();

this returns a console error

"SyntaxError: Unexpected token '{'. Parse error."

Why can i not write the if and else conditionals like this?

Please could someone explain the answers to these little problems so i can move on! thank you in advance and forgive me, i am very new to coding.

:: )

Hi again.. UPDATE and another QUESTION.

The reason I had the initial question is because of a codeschool.com function exercise which concluded as this...

function countE(){ var phrase = prompt("Which phrase would you like to examine?");

if (typeof(phrase) != "string"){
  alert("This is not a valid entry!");
  return false;
} else {

  var eCount = 0;
  for (var i = 0; i < phrase.length; i++){
      if (phrase.charAt(i) === 'e' || phrase.charAt(i) === 'E')
      eCount++;
      }
    }

      alert(eCount);
      return true;

}

countE()

So.. I wanted to test what is not a string, I wanted to get the alert "This is not a valid entry!".

But, if a prompt only returns a string then why is this << if (typeof(phrase) != "string") >> included in the function?

It will always return a string, as that's what prompt returns. And for you second question it's } else if (typeof input ==.. — Keith
– Keith, Commented Nov 11, 2016 at 22:46

Christos · Accepted Answer · 2016-11-11 22:45:47Z

3

what do i need to input to get the alert "That is not a string"? which kind of inputs are not strings?

The result of the prompt always is a string. So there isn't any input that you could provide to the prompt that could result in a different type. The prompt always returns a string.

For a more formal approach, please have a look here.

answered Nov 11, 2016 at 22:45

Christos

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1 Comment

James Over a year ago

Thanks Christos, I will reference MDN more often now. Please can you answer my further question?

jhack · Accepted Answer · 2016-11-16 15:16:08Z

1

For stringOrNot() you have a condition in your else statement. If you wanted to have a condition, use else if () {}.

function stringOrNot() {

var input = prompt('Input something here');

if (typeof input != 'string') {
    alert("That is not a string!");

  } else if (typeof input == 'string'){
    alert("That is a string!");
  }
}

Check below for more information

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/if...else

EDITED FOR QUESTION 2:

You include the check for the prompt because the it can also return a null value (when the user clicks exit). So when the user clicks Cancel, the "This is not a valid entry!" will appear.

edited Nov 16, 2016 at 15:16

answered Nov 11, 2016 at 22:47

jhack

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2 Comments

James Over a year ago

i figured out my issue. I use Safari. in Safari pressing esc or cancel alerts '0'. In Chrome - 'This is not a valid entry!' is alerted on esc or cancel. Do you know how I can get safari to work properly with this? Thanks

jhack Over a year ago

You could also add a check to return "This is not a valid entry" if eCount is 0.

Quentin · Accepted Answer · 2016-11-11 22:47:02Z

1

what do i need to input to get the alert "That is not a string"? which kind of inputs are not strings?

Things that are not strings include numbers, objects and booleans.

The return value of the prompt function will always be a string though. The purpose of the prompt function is to get a user entered string.

why can i not write the if and else conditionals like this?

if takes a condition. else is what happens if the condition is not met.

You are providing a condition to else, which doesn't make sense.

You can use another if statement though:

} else if (something) {

answered Nov 11, 2016 at 22:47

Quentin

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