JavaScript quirk 4: unknown variable names create global variables

[This post is part of a series on JavaScript quirks.]

Normally, JavaScript automatically creates a global variable if you use an unknown variable name:

    > function f() { foo = 123 }
    > f()
    > foo
    123

Thankfully, you get a warning in ECMAScript 5 strict mode [1]:

    > function f() { 'use strict'; foo = 123 }
    > f()
    ReferenceError: foo is not defined

Reference:

1. JavaScript’s strict mode: a summary