Understanding kind-of for JavaScript Type Checking

The kind-of library is a utility module that helps you determine the type of a given JavaScript value. It’s a handy tool for validating inputs and debugging code, ensuring robust and error-free applications. This guide will introduce you to the kind-of library and provide you with numerous API examples to make the most of this powerful utility.

Installation

  
  npm install kind-of
  

Basic Usage

After installing, you can easily use kind-of to check the type of any variable:

  
  const kindOf = require('kind-of');

  console.log(kindOf('Hello, world!')); //=> 'string'
  console.log(kindOf(123));            //=> 'number'
  console.log(kindOf(true));           //=> 'boolean'
  console.log(kindOf(null));           //=> 'null'
  console.log(kindOf(undefined));      //=> 'undefined'
  console.log(kindOf({}));             //=> 'object'
  console.log(kindOf([]));             //=> 'array'
  console.log(kindOf(() => {}));       //=> 'function'
  

Advanced Usage

kind-of can even distinguish between object types:

  
  console.log(kindOf(new Date()));                 //=> 'date'
  console.log(kindOf(new RegExp('ab+c')));         //=> 'regexp'
  console.log(kindOf(new Error('Something went wrong'))); //=> 'error'
  console.log(kindOf(new Map()));                 //=> 'map'
  console.log(kindOf(new Set()));                 //=> 'set'
  console.log(kindOf(Symbol('symbol')));          //=> 'symbol'
  

Custom Type Checking

You can even create custom objects and check their types:

  
  function Custom() {}
  const customObj = new Custom();

  console.log(kindOf(customObj)); //=> 'object'
  

Real-World Application

Here’s a simple app example that validates user inputs in a form:

  
  const kindOf = require('kind-of');

  function validateInput(input) {
    if (kindOf(input.name) !== 'string') {
      return 'Invalid name';
    }
    if (kindOf(input.age) !== 'number') {
      return 'Invalid age';
    }
    if (kindOf(input.email) !== 'string') {
      return 'Invalid email';
    }
    return 'Valid input';
  }

  const userInput = {
    name: 'John Doe',
    age: 30,
    email: 'john.doe@example.com'
  };

  console.log(validateInput(userInput)); //=> 'Valid input'
  

By using kind-of, you can ensure the data integrity and robustness of your applications while streamlining debugging and enhancing readability. Try incorporating kind-of in your JavaScript projects to see the benefits firsthand!

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