为什么`Array.length`，`Function.length`，`String.length`等返回1？

Question

While teaching my JavaScript class yesterday, my students and I came across some interesting functionality that I thought might be worth capturing in a question and the answer I've come to.

Typing Array.length in the JS console in chrome returns 1 .

Likewise, Function.length returns 1 . This is important because:

Every function in JavaScript is actually a Function object. (MDN JS Ref: Function)

Thus, Object.length and likely all other native objects will and should return 1 as the value of the length property.

So, finally why is this behavior occurring?

Answer 1

Function.length itself is the answer:

Specifies the number of arguments expected by the function. MDN JS Ref: Function.length

When we write Function.length we are asking the Function constructor to tell us the number of formal, named parameters ("optional" - ie non-formal - parameters are accessed via the arguments property in the function body). Because the Function constructor expects exactly 1 formal named parameter, the result is 1 :

new Function ([arg1[, arg2[, ... argN]],] functionBody)

functionBody is the single formal named parameter. Therefore Function.length is 1 .