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  3. Check if a Value is a String in JavaScript and Node.js

Check if a Value is a String in JavaScript and Node.js

Sometimes you want to ensure a given value is actually a string. For example, you may want to confirm a name property to be a string value.

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

Node.js Series Overview

  1. Increase the Memory Limit for Your Process
  2. Why You Should Add “node” in Your Travis Config
  3. Create a PDF from HTML with Puppeteer and Handlebars
  4. Create Your Own Custom Error
  5. Retrieve a Request’s IP Address in Node.js
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  1. Filter Data in Streams
  1. Get Number of Seconds Since Epoch in JavaScript
  2. Get Tomorrow’s Date in JavaScript
  3. Increase a Date in JavaScript by One Week
  4. Add Seconds to a Date in Node.js and JavaScript
  5. Add Month(s) to a Date in JavaScript or Node.js
  1. How to Run an Asynchronous Function in Array.map()
  2. How to Reset and Empty an Array
  3. for…of vs. for…in Loops
  4. Clone/Copy an Array in JavaScript and Node.js
  5. Get an Array With Unique Values (Delete Duplicates)
  6. Sort an Array of Integers in JavaScript and Node.js
  7. Sort a Boolean Array in JavaScript, TypeScript, or Node.js
  8. Check If an Array Contains a Given Value in JavaScript or Node.js
  9. Add an Item to the Beginning of an Array in JavaScript or Node.js
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  11. How to Exit and Stop a for Loop in JavaScript and Node.js (Coming soon)
  12. Split an Array Into Smaller Array Chunks in JavaScript and Node.js
  13. How to Get an Index in a for…of Loop in JavaScript and Node.js (Coming soon)
  14. How to Exit, Stop, or Break an Array#forEach Loop in JavaScript or Node.js (Coming soon)
  1. Callback and Promise Support in your Node.js Modules
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  3. Run Async Functions/Promises in Parallel
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  5. How to Fix “Promise resolver undefined is not a function” in Node.js or JavaScript
  6. Detect if Value Is a Promise in Node.js and JavaScript
  1. Human-Readable JSON.stringify() With Spaces and Line Breaks
  2. Write a JSON Object to a File
  3. Create a Custom “toJSON” Function in Node.js and JavaScript
  1. Extend Multiple Classes (Multi Inheritance)
  2. Retrieve the Class Name at Runtime in JavaScript and Node.js
  1. Generate a Random Number in Range With JavaScript/Node.js
  1. How to Merge Objects
  2. How to Check if an Object is Empty in JavaScript or Node.js
  3. How to CamelCase Keys of an Object in JavaScript or Node.js
  1. Get a File’s Created Date
  2. Get a File’s Last Modified/Updated Date
  3. How to Create an Empty File
  4. Check If a Path or File Exists
  5. How to Rename a File
  6. Check If a Path Is a Directory (Coming soon)
  7. Check If a Path Is a File (Coming soon)
  8. Retrieve the Path to the User’s Home Directory (Coming soon)
  9. How to Touch a File (Coming soon)

Determine If a Given Value Is a JavaScript String

JavaScript has the typeof operator. It returns the underlying type of a variable. This operator is useful when determining whether a given input value is a string:

/**
 * Determine whether the given `input` is a string.
 *
 * @param {*} input
 *
 * @returns {Boolean}
 */
function isString (input) {  
  return typeof input === 'string' && Object.prototype.toString.call(input) === '[object String]'
}

isString('Hello Marcus')  
// true

isString(123)  
// false

I guess in 99% of all cases you’re totally fine just using the typeof input === 'string' check.

Well, you may ask “why doing the extra Object.prototype.toString.call(input)call” then? You know, shit can go crazy in JavaScript! Sometimes you’re tinkering with prototypes of an instance or assigning a new value to the constructor property. This extra toString comparison is the layer ensuring you’re catching the 1% cases.

Use the @supercharge/strings Package

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

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

const isString = Str.isString('Hello')  
// true

Str.isString(null) // false  
Str.isString(1234) // false  
Str.isString([1, 2, 3]) // false  
// … and so on :)

Enjoy!

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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