javascript - (Why) does setting `array[NaN]` do nothing? -

i reading an article javascript prngs, , came across surprised me:

var = new array(); var b; a[b++] = 1; 

a [] , no exception thrown — the write array vanishes. try out in browser console if don’t believe me.

i didn't believe him, tried out in browser console (firefox 47):

» var = new array(); » var b; » a[b++] = 1  » ← array [  ] » b ← nan 

there several curious things going on here, in particular, i'm trying understand why statement a[b++] = 1 doesn't [appear to] anything.

taking top:

var = new array(); // 'a' empty array, plain ol' boring empty array, have been  // written = []; var b; // 'b' have no value , 'undefined'. 'console.log(b); // undefined' a[b++] = 1; // lets break above statement down pieces:   b++       // increment b one, undefined + 1 === nan a[   ]      // use nan property our array 'a'        = 1; // assign 1 property // ok? happened why 'a' still empty? console.log(a); // [] // way console show array printing numeric keys // in it, , looking @ length, eg: // var q = []; // q.length = 1; // console.log(q); // [undefined x 1]  // 'a' in our case there no numeric keys in [] printed. // did our value dissapear? // no. there: console.log('nan' in a); // true // and: (var prop in a) console.log(prop); // nan  // why feature?  // arrays extending objects have same properties em. console.log(a instanceof object); // true console.log(a instanceof array); // true