A way to think about a JavaScript universe.

Value Rules

  1. Everything is a value.

  2. Primitive values are immutable. There is only one value in the universe for each primitive value.

    Primitive values:

    • Undefined
    • Null
    • Boolean
    • Number
      • Special numbers: NaN, Infinity, -Infinity, -0
    • String
    • Symbol
    • BigInt

  3. Non-primitive values are mutable. A new value is created for each object literal or function expression evaluation.

    Non-primitive values:

    • Function
    • Object

  4. A variable or a property of object points to a value. Equality means they point to the same value.

Equality Rules

Object.is(a, b) is called “same value equality”. It can compare two objects or not. It is the same as a === b except for:

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Object.is(NaN, NaN) // true
NaN === NaN; // false (historical reason)

Object.is(0, -0) // false;
0 === -0 // true

The null Type

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// null has a typeof 'object' (this is a historical bug in JavaScript)
typeof null === "object";

Semicolon Rule

Add a leading semicolon when a line starts with one of the following:

+ - / [ (

In practice:

[ (