In last article I explained how to use variables in JavaScript but I also mentioned undefined, here I will try my best to explain to you what is undefined, what is the difference with null, why both are existing and I will give you some examples.
Undefined
In JavaScript, undefined means a variable has been declared but has not yet been assigned a value or assigned undefined :
// console.warn(notDeclared); // Impossible not defined
console.warn(typeof notDeclared); // logs : undefined
let test;
console.warn(test); // logs : undefined
console.warn(typeof test); // logs : undefined
const testConst = undefined;
console.warn(testConst); // logs : undefined
console.warn(typeof testConst); // logs : undefined
// Impossible, missing initializer in const declaration
/* const testConst2;
console.warn(testConst2); // Broken code
console.warn(typeof testConst2); // Broken code */
Null
Unlike undefined, null is an assignment value, null can be assigned to a variable as a representation of no value :
const test = null;
console.warn(test); // logs : null
console.warn(typeof test); // logs : object
As you can see undefined and null are very different undefined is a type itself, and null seems like an object but no it is in reality a primitive value
To understand why null is not an object I will quote a sentence from the book Professional JavaScript for Web Developers :* "You may wonder why the typeof operator returns 'object' for a value that is null. This was actually an error in the original JavaScript implementation that was then copied in ECMAScript. Today, it is rationalized that null is considered a placeholder for an object, even though, technically, it is a primitive value."*
More examples
Sometimes JavaScript is fun, or horrible depending on how you feel 🤒
console.warn(null === undefined); // logs : false
console.warn(null == undefined); // logs : true
console.warn(null === null); // logs : true
// null = 'value'; // Impossible ReferenceError
// let undefined = 'value'; Impossible trick :
// Identifier 'undefined' has already been declared
// /!\ DON'T DO THIS /!\
(function () {
var undefined = 'wtf';
console.warn(undefined, typeof undefined); // logs : wtf string
})();
// /!\ DON'T DO THIS /!\
(function (undefined) {
console.warn(undefined, typeof undefined); // logs: wtf2 string
})('wtf2');
That's it for undefined and null !
🚀 In my next article you can learn how to use conditions in JavaScript 🚀