let keyword in the for loop
ECMAScript 6's let
is supposed to provide block scope without hoisting headaches. Can some explain why in the code below i
in the function resolves to the last value from the loop (just like with var
) instead of the value from the current iteration?
"use strict";
var things = {};
for (let i = 0; i < 3; i++) {
things["fun" + i] = function() {
console.log(i);
};
}
things["fun0"](); // prints 3
things["fun1"](); // prints 3
things["fun2"](); // prints 3
According to MDN using let
in the for
loop like that should bind the variable in the scope of the loop's body. Things work as I'd expect them when I use a temporary variable inside the block. Why is that necessary?
"use strict";
var things = {};
for (let i = 0; i < 3; i++) {
let index = i;
things["fun" + i] = function() {
console.log(index);
};
}
things["fun0"](); // prints 0
things["fun1"](); // prints 1
things["fun2"](); // prints 2
I tested the script with Traceur and node --harmony
.
Solution 1:
squint's answer is no longer up-to-date. In ECMA 6 specification, the specified behaviour is that in
for(let i;;){}
i
gets a new binding for every iteration of the loop.
This means that every closure captures a different i
instance. So the result of 012
is the correct result as of now. When you run this in Chrome v47+, you get the correct result. When you run it in IE11 and Edge, currently the incorrect result (333
) seems to be produced.
More information regarding this bug/feature can be found in the links in this page;
Since when the let
expression is used, every iteration creates a new lexical scope chained up to the previous scope. This has performance implications for using the let
expression, which is reported here.
Solution 2:
I passed this code through Babel so we can understand the behaviour in terms of familiar ES5:
for (let i = 0; i < 3; i++) {
i++;
things["fun" + i] = function() {
console.log(i);
};
i--;
}
Here is the code transpiled to ES5:
var _loop = function _loop(_i) {
_i++;
things["fun" + _i] = function () {
console.log(_i);
};
_i--;
i = _i;
};
for (var i = 0; i < 3; i++) {
_loop(i);
}
We can see that two variables are used.
In the outer scope
i
is the variable that changes as we iterate.-
In the inner scope
_i
is a unique variable for each iteration. There will eventually be three separate instances of_i
.Each callback function can see its corresponding
_i
, and could even manipulate it if it wanted to, independently of the_i
s in other scopes.(You can confirm that there are three different
_i
s by doingconsole.log(i++)
inside the callback. Changing_i
in an earlier callback does not affect the output from later callbacks.)
At the end of each iteration, the value of _i
is copied into i
. Therefore changing the unique inner variable during the iteration will affect the outer iterated variable.
It is good to see that ES6 has continued the long-standing tradition of WTFJS.