How and why does ‘a'[‘toUpperCase’]() in JavaScript work?

To break it down.

• .toUpperCase() is a method of String.prototype
• 'a' is a primitive value, but gets converted into its Object representation
• We have two possible notations to access object properties/methods, dot and bracket notation

So

'a'['toUpperCase'];

is the access via bracket notation on the property toUpperCase, from String.prototype. Since this property references a method, we can invoke it by attaching ()

'a'['toUpperCase']();