language features - Are there legitimate uses for JavaScript's "with" statement? -

alan storm's comments in response answer regarding with statement got me thinking. i've seldom found reason use particular language feature, , had never given thought how might cause trouble. now, i'm curious how might make effective use of with, while avoiding pitfalls.

where have found with statement useful?

another use occurred me today, searched web excitedly , found existing mention of it: defining variables inside block scope.

background

javascript, in spite of superficial resemblance c , c++, not scope variables block defined in:

var name = "joe"; if ( true ) {    var name = "jack"; } // name contains "jack" 

declaring closure in loop common task can lead errors:

for (var i=0; i<3; ++i) {    var num = i;    settimeout(function() { alert(num); }, 10); } 

because loop not introduce new scope, same num - value of 2 - shared 3 functions.

a new scope: let , with

with introduction of let statement in es6, becomes easy introduce new scope when necessary avoid these problems:

// variables introduced in statement  // scoped each iteration of loop (let i=0; i<3; ++i) {    settimeout(function() { alert(i); }, 10); } 

or even:

for (var i=0; i<3; ++i) {    // variables introduced in statement     // scoped block containing it.    let num = i;    settimeout(function() { alert(num); }, 10); } 

until es6 universally available, use remains limited newest browsers , developers willing use transpilers. however, can simulate behavior using with:

for (var i=0; i<3; ++i) {    // object members introduced in statement     // scoped block following it.    ({num: i})    {       settimeout(function() { alert(num); }, 10);    } } 

the loop works intended, creating 3 separate variables values 0 2. note variables declared within block not scoped it, unlike behavior of blocks in c++ (in c, variables must declared @ start of block, in way similar). behavior quite similar let block syntax introduced in earlier versions of mozilla browsers, not adopted elsewhere.