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I am trying out 'strict mode' and using keyword let inside of a for-loop like this: 

  (function() {

    "use strict";

    var numbers = [1,2,3,4,5];
    var value;


    for (let i = j = 0; i < numbers.length; i++) {
      j += numbers[i];
      value = j;
    }
      console.log ("\tThe reduced value is: ", value);    
  })();

This produces an Uncaught ReferenceError: j is not defined, but by removing 'use strict' the code runs fine.

The code is reduced subset of my jsfiddle

Eggs
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  • try this way, you should assign `j` before using it,`for (let j, i = j = 0; i < numbers.length; i++) {` – Aswin Ramesh Feb 07 '18 at 05:13
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    (It doesn’t run *fine* without strict mode – it quietly does the wrong thing.) – Ry- Feb 07 '18 at 05:16
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    @AswinRamesh: While `let j, i = j = 0` is perfectly correct, I'd rather do `let i = 0, j = 0`. Computer doesn't care, colleagues will be grateful. :P – Amadan Feb 07 '18 at 05:18
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    As a side note, value = [1,2,3,4,5].reduce((sum, next) => sum+next, 0); does the same thing as the loop above. – keithlee96 Feb 07 '18 at 05:21
  • @Ryan. Quietly creating a global var j without strict mode I get it now. – Eggs Feb 07 '18 at 05:44

2 Answers2

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The problem is as you would expect, the initialiser statement in your for loop:

let i = j = 0;

This statement is identical to:

let i = (j=0);

which is the same as:

j = 0; // would throw : Uncaught ReferenceError: j is not defined
let i = j;

If you want to fix the strict-mode error, you need to explicitly declare j as a variable with let j = 0;

It works in sloppy mode due to hoisting, which will create a variable j in the global scope. (https://www.w3schools.com/js/js_hoisting.asp)

Edit: Bergi is right, I mixed up the concepts, it becomes available in the global scope due to the the scope chain, not hoisting. If a variable is undeclared in the local scope, it looks at the outer lexical scope for the variable. If it's still not defined there, it keeps going up the scope chain until it hits the global scope. If it's still not defined there, then it automatically gets defined in the global scope and that is the variable you reference every time you use j.

keithlee96
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    You might want to add why it "works" in sloppy mode: it just creates a global `j` variable. Which hardly anyone would want - so everyone uses strict mode. – Bergi Feb 07 '18 at 05:16
  • A reference link for it: https://stackoverflow.com/questions/1470488/what-is-the-purpose-of-the-var-keyword-and-when-to-use-it-or-omit-it – Rajesh Feb 07 '18 at 05:21
  • No, this is not what hoisting means. – Bergi Feb 07 '18 at 06:05
  • @Bergi How would you define hoisting? – keithlee96 Feb 07 '18 at 22:27
  • @keithlee96 Hoisting refers to the visibility of a variable being the whole scope it is *declared* in (instead of just the section after the declaration) and the initialisation behaviour when entering said scope. But `j = 0` isn't even declared anywhere, it's just an assignment, so the term "hoisting" cannot apply at all. – Bergi Feb 08 '18 at 05:02
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The construction 

let i = j = 0

has actually nothing to do with the keyword let. Yet

Let's look at what it does under the hood:

let i // declares i
j = 0 // assigns a variable j to 0 that is not declared
      // If it does not exist it creates the variable on the `window` object.
i = j // in fact i = window.j

To make it right use 

let i = 0, j = 0
Bergi
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smnbbrv
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