How to check for a string in JavaScript
When checking for strings, typeof works great unless someone uses a primitive object wrapper, which is considered bad code.
Check for a primitive string
Some things are a little tricky to check for in JavaScript, like checking for NaN, but thankfully strings are a lot easier to deal with.
When checking if a value is a string in JavaScript, typeof works great:
Well, typeof does not work as expected in the case of strings created with new String(), which is what is called the primitive object wrapper.
Creating “object-wrapped” strings like this is seldom used and generally recommended against, as I explore in the next section.
Why are primitive object wrappers bad?
Because just about everyone online says JavaScript object wrappers are bad, confusing, and should go away. For example:
- The Google JavaScript Style Guide says to never, ever use primitive object wrappers, you naughty developers you.
- Douglas Crockford publicly recommended that primitive object wrappers be deprecated — that their use be officially discouraged.
In fact, I have never seen anyone recommending their use. Primitive object wrappers appear to be pretty universally reviled.
In the next section, I review what one needs to know about them.
How not to use object wrappers
Really, the only time time wrappers come up is in “Gotcha!” style interview questions, usually regarding type-checking using typeof.
In fact, of the following three ways of invoking a string, I think almost all JavaScript programmers would only even be aware of the first one.
The other two syntaxes should be generally be avoided as confusing:
Object wrappers are confusing
“[E]xplicitly creating wrappers using the new operator is considered a bad practice. On the other hand, calling the function without the new operator is perfectly valid as it just tries to convert the input into the corresponding primitive type and returns a primitive value.”
— Vojtěch Růžička on his blog
In addition to changing what typeof returns, there is another thing about primitive object wrappers that is confusing: type coercion.
The same function can be used without the keyword new in order to coerce a value to that primitive type. Here is what that looks like:
So, using the wrapper function without the new keyword is a useful way of coercing a value into a primitive type.
On the other hand, using the new keyword creates an object-wrapped primitive, which is considered bad mojo.
Why do wrappers exist if they are bad?
Basically, these primitive object wrappers only exist in JavaScript to allow primitive types to act like objects.
JavaScript automatically calls the primitive object wrapper behind the scenes when the code references a prototype function or property.
For example, this occurs when checking the length of a string using the String.prototype.length property or replacing all substrings within a string using the String.prototype.replace() function:
In both of these cases, JavaScript calls the object wrapper under the hood in order to be able to reference the object’s properties and methods.
Proofing against an object wrapper
What if your code happens to be riddled with tons of primitive object wrappers? In that case, how do you check for a string anyway?
The tip is to know about two additional JavaScript features — the instanceof keyword and the Object.prototype.toString.call() function.
Here is how to use those for an improved isString() function:
Conclusion
Generally speaking, typeof will get you pretty far in your day-to-day code when checking for a string.
Usually, objected wrapped Strings only exist as a JavaScript quiz question, but they are easy to handle if you need to check for them.
For object wrapped primitives, the instanceof keyword and the Object.prototype.toString.call() function will reveal a value’s type.
Additional resources
- Nikhil Swain discussed working with JavaScript strings in The Startup:
- Samantha Ming talked about converting values to strings in Daily JS:
- Trey Huffine wrote about many string methods in gitconnected:
- The CodeDraken covered types and data structures at codeburst:
- Sonya Moisset demonstrates how to reverse a string at freeCodeCamp:
Dr. Derek Austin is the author of Career Programming: How You Can Become a Successful 6-Figure Programmer in 6 Months, now available on Amazon.
