Ecmascript 2015: Const in for Loops

ECMAScript 2015: const in for loops

The following for-of loop works:

for (const e of a)

The ES6 specification describes this as:

ForDeclaration : LetOrConst ForBinding

http://www.ecma-international.org/ecma-262/6.0/index.html#sec-for-in-and-for-of-statements-static-semantics-boundnames

The imperative for loop will not work:

for (const i = 0; i < a.length; i += 1)

This is because the declaration is only evaluated once before the loop body is executed.

http://www.ecma-international.org/ecma-262/6.0/index.html#sec-for-statement-runtime-semantics-labelledevaluation

for...of loop. Should I use const or let?

Why use const and when to use let in for...of loops?

If there are no assignments to the identifier within the loop body, it's a matter of style whether you use let or const.

Use const if you want the identifier within the loop body to be read-only (so that, for instance, if someone modifies the code later to add an assignment, it's a proactive error). Use let if you want to be able to assign to it (because you have an assignment in your code, or you want someone to be able to add one later without changing the declaration).

You can do this with for-of and for-in loops. A for loop's control variable is normally not constant (since in the normal case you update it in the "update" clause of the for; if you don't, for may be the wrong loop to use), so you normally use let with it.

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For clarity, here's an example with an assignment within the loop body:

for (let str of ["a", " b", " c "]) {
    str = str.trim();
//  ^^^^^----- assignment to the identifier
    console.log(`[${str}]`);
}

Why can a for..of / for..in loop use const while a normal for loop can only use let or var for its variable in JS?

No the for(...of...) loop won't get an error. Here is why:

In the second for loop, you are editing the constant variable which is NOT allowed. This throws a TypeError. In the first for loop, it doesn't matter what type of variable assignment you use. The variable (const,let, var, or no identifier) calls on the iterator which creates a sort of isolated temporary scope. This is why you can't access the variable outside of the for loop. For example:

const example = 1;
example = 2;
//TypeError

for (const someVar of Array(5)) someVar = 12;
//TypeError

{
    const num = 200;
}

const num = 150;
//This is fine because of scope

Why does const work in some for-loops in JavaScript?

for (const property in object) works because with each iteration you get a new variable, which is scoped only to that iteration. You can easily check that by using a closure inside a loop:

for (const property in {a: 1, b: 2}) {  setTimeout(() => {    console.log(property);  }, 100);}

Can for-loops counter be const declaration in ES2015?

I tested it (in Firefox) and the const binding works according to spec:

let i = 0;for (const len = 3; i < len; i++) {  console.log(i);}
// From https://kangax.github.io/compat-table/es6/#test-constfor (const baz = 0; false;) {}
// Yay, a const counter! ...uhfor (const counter = {i: 0}; counter.i < 3; counter.i++) {  console.log(counter.i);}

Using const as loop variable in for loop

It works in Stackblitz because it is running traspiled code:

AppComponent.prototype.test = function () {
    var _loop_1 = function (i) {
        setTimeout(function () {
            console.log(i);
        }, 100 * i);
    };
    for (var i = 0; i < 5; i++) {
        _loop_1(i);
    }
};

It won't work if you add a snippet here because it is not transpiled

for (const i = 0; i < 5; i++) {  setTimeout(function() {    console.log(i)  }, 100 * i);}

Why JavaScript const works well with for in loop

Why JavaScript const works same as let in for in loop?

By definition, const is block scoped like let.

Then why the const num value getting updated in each iteration of the for in?

It isn't. Since it is block scoped, each time you go around the loop the old constant drops out of scope and you create a new one.

Why `const` value changed inside of `for...in` and `for...of` loop?

Every iteration of a loop has its own block scope.

 for(let i = 0; i < 10; i++)
   setTimeout(() => console.log(i), 1);

That creates 10 seperate scopes, thats why it logs 10 different numbers. Therefore you can also declare constants in these different scopes.

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