JavaScript history: undefined

Two tweets by Brendan Eich shed light on the history of JavaScript having both undefined and null [1].

The first version of JavaScript did not have exception handling, which is why JavaScript so often converts automatically [2] and/or fails silently (tweet).

JavaScript copied Java’s approach of partitioning values into primitives and objects [3]. null is the value for “not an object”. The precedent from C (but not from Java) is to convert null to 0 (C has pointers, not references and lets you perform arithmetic with pointers).

Remaining problem: In JavaScript, each variable can hold both primitives and objects. In Java, a variable’s static type limits it to either kind of value. We therefore need a value for “neither a primitive nor an object”. That value could be null, but at the time, Eich wanted something that wasn’t “reference-y” (associated with objects) and did not convert to 0 (tweet). Now you know why undefined and null are converted to different numbers:

    > Number(undefined)
    NaN
    > Number(null)
    0

References:

1. JavaScript quirk 2: two “non-values” – undefined and null
2. JavaScript quirk 1: implicit conversion of values
3. Categorizing values in JavaScript