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  3. Check if a Value is a Symbol in JavaScript or Node.js

Check if a Value is a Symbol in JavaScript or Node.js

JavaScript contains the global Symbol object allowing you to create unique symbol values. You may use symbols to add a property to an object that doesn’t collide with other keys because symbols are hidden from the usual access patterns.

This tutorial shows you how to detect whether a given value is a symbol in JavaScript.

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Determine If a Given Value Is a JavaScript Symbol

You can create symbol primitives using the Symbol() function or the static Symbol.for(<key>) method. The difference between both functions is that Symbol always returns a new symbol value. Symbol.for always returns the same symbol value when calling it with the same key.

In your application, you may need to detect whether a given value is a symbol. Here’s a utility function isSymbol that returns true if a given input is a symbol, false otherwise:

/**
 * Determine whether the given `input` is a symbol.
 *
 * @param {*} input
 *
 * @returns {Boolean}
 */
function isSymbol (input) {  
  return Object.prototype.toString.call(input) === '[object Symbol]'
}

isSymbol(Symbol.for('marcus')) // true  
isSymbol(Object(Symbol.for('marcus'))) // true

isSymbol(null) // false  
isSymbol(1234) // false

This isSymbol method is a mature check for primitive symbol values and also for wrapped symbols.

Be Careful Using “typeof === 'symbol'”

In case you saw a symbol check like typeof val === 'symbol', that’s a valid check for most use cases.

The situation where a typeof check fails is a wrapped symbol. JavaScript supports wrapped symbols using Object(Symbol('marcus')). Using the typeof check would return false for a wrapped symbol instead of true.

Use the @supercharge/strings Package

I’m the maintainer of the @supercharge/strings package providing convenient string utilities. It provides the solid Str.isSymbol method determining whether a given value is a symbol:

const Str = require('@supercharge/strings')

Str.isSymbol(Symbol.for('marcus')) // true  
Str.isSymbol(Object(Symbol.for('marcus'))) // true

Str.isSymbol(null) // false  
Str.isSymbol(1234) // false

That’s it!

Mentioned Resources

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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