How to Check Variable Types in TypeScript

Ferenc Almasi • 2022 September 19 • 📖 3 min read

To check variable types in TypeScript, use the typeof operator which returns the type of a variable as a string. Take the following code as an example:

const variable1: string | number = '42'
const variable2: string | number = 42

// This will return "string"
console.log(typeof variable1)

// This will return "number"
console.log(typeof variable2)

// This is called typeguard
if (typeof variable1 === 'string') {
    variable1.toLowerCase()
}
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Using a type check to execute code is called a typeguard, and can ensure that you work with the correct type. For example, we don't have a toLowerCase method on numbers, but we have it for strings. The typeof operator can have the following return values:

typeof '1'       // Returns "string"
typeof 1         // Returns "number"
typeof NaN       // Returns "number"
typeof true      // Returns "boolean"
typeof undefined // Returns "undefined"
typeof Symbol()  // Returns "symbol"
typeof []        // Returns "object"
typeof null      // Returns "object"
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If you are interested in why null would be an object type, I recommend checking out "The history of typeof null" by 2ality. Another interesting behavior is that if you use the typeof operator twice after each other, you will always get a "string" type.

typeof typeof 1    // Returns "string"
typeof typeof true // Returns "string"
typeof typeof []   // Returns "string"
typeof typeof null // Returns "string"
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We can also use the typeof operator to dynamically assign the type for other variables. This can be done in the following way:

// Assigning type to a variable using the typeof operator
const variable1: string = 'string'
const variable2: typeof variable1 = 42 // This will throw an error: "Type 'number' is not assignable to type 'string'"
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Using instanceof

It's also worth noting that if you need to check whether your variable is a type of a class, you need to use the instanceof keyword instead of the typeof operator.

class Fruit {}

const fruit = new Fruit()

// This will return true
if (fruit instanceof Fruit) {
    // Call method only available on Fruit classes
    fruit.grow()
}
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Since the fruit variable was created as a new instance of the Fruit class, the if statement will be evaluated to true.

Using the instanceof operator will always return a boolean value (true or false). 

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