Check if a Variable is a String using JavaScript

# Check if a Variable is a String in JavaScript

Use the typeof operator to check if a variable is a string, e.g. if (typeof variable === 'string').

If the typeof operator returns "string", then the variable stores a string.

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const variable = 'bobbyhadz.com';

if (typeof variable === 'string') {
  // 👇️ this runs
  console.log('✅ the value is of type string');
} else {
  console.log('⛔️ the value is NOT of type string');
}

We used the typeof operator to check if a variable is a string.

The operator returns a string that indicates the type of the value.

Here are some examples:

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console.log(typeof ""); // 👉️ "string"
console.log(typeof "hello"); // 👉️ "string"
console.log(typeof "42"); // 👉️ "string"
console.log(typeof function () {}); // 👉️ "function"
console.log(typeof null); // 👉️ "object"
console.log(typeof []); // 👉️ "object"
console.log(typeof {}); // 👉️ "object"
console.log(typeof 0); // 👉️ "number"

If the variable stores a string, the equality comparison will return true and the if block will run, otherwise, the else block will run.

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if (typeof variable === 'string') {
  console.log('✅ type is string');
} else {
  // 👇️ this runs
  console.log('⛔️ type is NOT string');
}

We used the typeof operator with a variable that hasn't been declared, so it returned a sting of "undefined".

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console.log(typeof doesNotExist); // 👉️ "undefined"

# No automatic type conversion is performed

Note that no automatic type conversion is performed. If your variable contains a number that is wrapped in a string, the typeof operator still returns "string".

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console.log(typeof '42'); // 👉️ string

The only scenario where the typeof operator wouldn't detect a string is if the string is created using the new String() constructor.

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const str = new String('hello world');

console.log(typeof str); // 👉️ "object"

Note that the String() constructor is not the same as the String object. The difference being the new keyword.

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console.log(typeof new String('hello')); // 👉️ "object"
console.log(typeof String('hello')); // 👉️ "string"

The String() constructor (with the new keyword), produces an instance of the String type and should never be used.

Whereas, the String object is used to convert a value to a string.

The typeof operator is all you need to check if a variable stores a string.

# Defining a reusable function

If you have to check if a variable stores a string often, define a reusable function.

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function isString(value) {
  return typeof value === 'string';
}

console.log(isString('abc')); // 👉️ true
console.log(isString('')); // 👉️ true
console.log(isString(42)); // 👉️ false
console.log(isString([])); // 👉️ false
console.log(isString(null)); // 👉️ false

const variable = 'bobbyhadz.com';

if (isString(variable)) {
  // 👇️ this runs
  console.log('The variable stores a string');
} else {
  console.log('The variable does NOT store a string');
}

The isString() function takes a value as a parameter and returns true if the value is of type string and false otherwise.

# Check if a Variable is a String using lodash

First, make sure you have lodash installed by running the following command from your terminal.

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# 👇️ initialize package.json if you don't have one
npm init -y

npm install lodash

Now you can import and use the isString() method to check if a value is a string.

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import _ from 'lodash';

console.log(_.isString('hello')); // 👉️ true
console.log(_.isString('')); // 👉️ true
console.log(_.isString(true)); // 👉️ false
console.log(_.isString(false)); // 👉️ false
console.log(_.isString(null)); // 👉️ false

const variable = 'bobbyhadz.com';

if (_.isString(variable)) {
  // 👇️ this runs
  console.log('The variable is a string');
} else {
  console.log('The variable is NOT a string');
}

The lodash.isString() method returns true if the supplied value is a string and false otherwise.

Even if you already use lodash in your project, I would advise against using the isString() method as the typeof operator is sufficient and doesn't require you to use an external library.

# Check if a Variable is a String using Object.prototype.toString.call()

Alternatively, you can use the Object.prototype.toString.call() method.

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const variable = 'bobbyhadz.com';

if (
  Object.prototype.toString.call(variable) === '[object String]'
) {
  // 👇️ this runs
  console.log('The variable is a string');
} else {
  console.log('The variable is NOT a string');
}

Here is an example of defining a reusable function.

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function isString(value) {
  return (
    Object.prototype.toString.call(value) === '[object String]'
  );
}

console.log(isString('hello')); // 👉️ true
console.log(isString('')); // 👉️ true
console.log(isString(null)); // 👉️ false

The Object.prototype.toString.call() method can be used to stringify a value.

When a string is stringified, the string '[object String]' is returned.

You shouldn't have to use this approach as it relies on the language's internals and is unintuitive.

# Don't use the instanceof operator to check if a value is a string

You should never use the instanceof operator to check if a value is of type string.

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// ⛔️ BAD - don't do this
function isString(value) {
  return value instanceof String;
}

console.log(isString('hello')); // 👉️ true
console.log(isString('')); // 👉️ true
console.log(isString(null)); // 👉️ false
console.log(isString(new String('hello'))); // 👉️ true

The syntax for the instanceof operator is object instanceof constructor.

The operator returns true if the prototype property of the constructor appears anywhere in the prototype chain of the object.

Notice that the operator returns true only for object-wrapped strings.

These are strings that were created with the new String() constructor.

You should never use the new String() constructor in your code, so you should never have to use the instanceof operator to check if a value is a string.

The typeof operator is all you need in order to check if a value stores a string in JavaScript.

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const variable = 'bobbyhadz.com';

if (typeof variable === 'string') {
  // 👇️ this runs
  console.log('✅ the value is of type string');
} else {
  console.log('⛔️ the value is NOT of type string');
}

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Check if Variable is equal to Multiple Values in JavaScript
• Check if a Variable is equal to True in JavaScript
• Check if a Variable is Not NULL in JavaScript
• Check if a Variable is of type Error in JavaScript