JavaScript: Check if Variable Is a String

Introduction

JavaScript supports a variety of data types such as strings, numbers, floats, etc. A string is a collection of characters such as "John Doe". Typically, you create them by enclosing characters in double or single quotes. Alternatively, you can make a string by using the new String() constructor:

let myString = 'John Doe';
let myString2 = new String("John Doe");

When performing specific operations, you may face a situation that demands you to verify that a specific variable is a string before processing it - lest an error be thrown. We'll cover that situation in this article! First, we'll take a look at how to check if a particular variable is a string in JavaScript and then show you an alternative approach that uses the Lodash library.

Standard Solution - Using typeof Operator

In JavaScript, the typeof operator is the most used method to check the type of any variable. Alternatively, you can use the typeof() method:

let myString = 'John Doe';

typeof myString;  // string
typeof(myString); // string

If used with a string, the typeof operator returns "string". Let's create a simple example to confirm this:

let myString = "John Doe";

if (typeof myString === "string") {
    console.log("This variable is a string");
} else {
    console.log("This variable is not a string");
}

Indeed, the myString is a string:

This variable is a string

Note: Even if the variable contains a number that is wrapped in single/double quotes, it still would be considered a string.

One interesting problem with the typeof operator is that it doesn't recognize strings created using the new String() constructor. The new keyword creates a new JavaScript object that is the instance of the String type. Therefore, the typeof operator won't correctly recognize strings created using the new String() constructor:

let myString = new String('John Doe');

console.log(typeof myString); // "object"

In this case, instead of the typeof operator, we need to use the instanceof operator - it can detect that the object created with the new String() constructor is the instance of the String type:

let myString = new String("John Doe");

if (myString instanceof String) {
    console.log("This variable is a string");
} else {
    console.log("This variable is not a string");
}

Since the myString is a string, this code will produce the following output:

This variable is a string

Using Lodash Library

If you're already using the Lodash library in your project, there's no harm in using it for checking whether a variable is a string or not! It's absolutely not necessary to have a dependency if we don't need Lodash for something else, but, if we already have that dependency, we can make use of the _.isString() method, which returns true if the specified value is a string primitive or a String object, making it fit for both explicitly and implicitly created strings:

let myString = new String("John Doe");

if (_.isString(myString)) {
    console.log("This variable is a string");
} else {
    console.log("This variable is not a string");
}

Output:

This variable is a string

Conclusion

In this article, we've learned how to check if a variable is a string in JavaScript. Also, we've learned how this works with an external library like Lodash.