JavaScript为什么getItem（）有两种返回类型？

Question

I am currently following a basic JavaScript Course and there was an example, that confused me, because it seems that the getItem() function can have two return types.

if(!localStorage.getItem('name')) {
  setUserName();
} else {
  var storedName = localStorage.getItem('name');
  myHeading.textContent =storedName;
}

So in the if condition getItem returns true or false depending on wether there is an Item stored in the localStorage or not. And later the return value is stored in a variable as a String to output it on the website(in the "else" part). I googled for this and didn't find anything, but maybe I searched wrong, so I will be happy for any help I can get here. I don't know, if this code is enough to understand what I mean, but I will post more later on if necessary.

Answer 1

getItem returns:

A DOMString containing the value of the key. If the key does not exist, null is returned.

And what is a DOMString ?

A DOMString is a UTF-16 String. As JavaScript already uses such strings, DOMString is mapped directly to a String.

So, it returns a string . Strings (unless they are empty) are considered "truthy" in JS :

In JavaScript, a truthy value is a value that is considered true when evaluated in a Boolean context. All values are truthy unless they are defined as falsy (ie, except for false, 0, "", null, undefined, and NaN).

So, in other words, if it returns a non-empty string, then the if statement considers it true. If not, it's considered false.

This also means the conclusion you drew, that getItem has "two return types" is incorrect. It has a single return type, and that type can be evaluated for truthiness.

Answer 2

Storage.getItem()

The getItem() method of the Storage interface, when passed a key name, will return that key's value.

Return value

A DOMString containing the value of the key. If the key does not exist, null is returned.

---

if(!localStorage.getItem('name')) {
  setUserName();
} else {
  var storedName = localStorage.getItem('name');
  myHeading.textContent =storedName;
}

Your snippet will check if the local storage contains an item for the key 'name'. It will not return a boolean (true/false) but the object itself. If there's no value for the key, it will return 'null'.

localStorage.getItem('name') returns:

• Null : The script will execute the function setUserName() .
• Any value : The script will save the value under a variable called storedName .