What is the difference between null and undefined in JavaScript?

Question

I want to know what the difference is between null and undefined in JavaScript.

Answer 1

tl;dr

Use null for set a variable you know it is an Object.

Use undefined for set a variable whose type is mixed.

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This is my usage of both 5 primitives and Object type, and that explain the difference between « use case » of undefined or null.

String

If you know a variable is only a string while all lifecycle, by convention, you could initialize it, to "":

("") ? true : false; // false
typeof ""; // "string";
("Hello World") ? true : false; // true
typeof "Hello World"; // "string"

Number

If you know a variable is only a number while all lifecycle, by convention, you could initialize it, to 0 (or NaN if 0 is an important value in your usage):

(0) ? true : false; // false
typeof 0; // "number";
(16) ? true : false; // true
typeof 16; // "number"

or

(NaN) ? true : false; // false
typeof NaN; // "number";
(16) ? true : false; // true
typeof 16; // "number"

Boolean

If you know a variable is only a boolean while all lifecycle, by convention, you could initialize it, to false:

(false) ? true : false; // false
typeof false; // "boolean";
(true) ? true : false; // true
typeof true; // "boolean"

Object

If you know a variable is only an Object while all lifecycle, by convention, you could initialize it, to null:

(null) ? true : false; // false
typeof null; // "object";
({}) ? true : false; // true
typeof {}; // "object"

Note: the smart usage off null is to be the falsy version of an Object because an Object is always true, and because typeof null return object. That means typeof myVarObject return consistent value for both Object and null type.

All

If you know a variable has a mixed type (any type while all lifecycle), by convention, you could initialize it, to undefined.