How to Check if a Variable is a String in JavaScript

Last updated on January 28, 2023
How to Check if a Variable is a String in JavaScript

To check if a variable is a string in JavaScript, use the typeof operator, i.e., if (typeof variable === 'string'). If the typeof variable returns 'string', then the variable is a string. Otherwise, it is not a string.

For example:

const variable = 'Coding Beauty';

if (typeof variable === 'string') {
  // ūüĎá this runs
  console.log('Variable is string');
} else {
  console.log('Variable is not string');
}

The typeof operator returns a string that indicates the type of its operand. For primitives, it returns the exact type, e.g., 'number', 'boolean', 'string', etc, for functions, it returns 'function', and for complex or user-defined types, it returns 'object'.

console.log(typeof true); // 'boolean'

console.log(typeof 'Coding Beauty'); // 'string'

console.log(typeof 100); // 'number'

console.log(typeof new Promise(() => {})); // 'object'

console.log(typeof function () {}); // 'function'

console.log(typeof Symbol()); // 'symbol'

console.log(typeof (() => {})); // 'function'

console.log(typeof []); // 'object'

console.log(typeof {}); // 'object'

console.log(typeof undefined); // 'undefined'

typeof undeclared_variable is 'undefined'?

Note that if you mistakenly use typeof on a variable that hasn't been declared, typeof will not throw an error:

if (typeof undeclared === 'string') {
  console.log('Variable is string');
} else {
  console.log('Variable is NOT string');
}

When used on variables that haven't been declared, typeof returns the string 'undefined':

console.log(typeof undeclared); // 'undefined'

typeof for string wrapper objects

The one instance where typeof won't work is when the string was created as a wrapper object i.e., using the String() constructor with the new operator.

For such strings, it will return the 'object' string, instead of 'string'.

const strObj = new String('Coding Beauty');

console.log(typeof strObj); // 'object'

However, it's not a good practice to use wrapper objects, as they serve no use. Wrapper objects are only used internally; when JavaScript auto-boxes a primitive value into an object to access a property on that object.

typeof null is 'object'?

Note that when typeof is used on the null value, it returns the string 'object'

console.log(typeof null) // 'object'

This is because, as stated in the MDN documentation:

In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Consequently, null had 0 as type tag, hence the typeof return value "object". (reference)

typeof NaN is 'number'?

Also, typeof NaN results in 'number':

console.log(typeof NaN); // 'number'

Because as stated in this StackOverflow answer and the spec, NaN is considered a numeric type in JavaScript.

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See also