According to our style guide, we default to following Google's style guide which makes the following recommendation:

Iterating over Node Lists

Node lists are often implemented as node iterators with a filter. This means that getting a property like length is O(n), and iterating over the list by re-checking the length will be O(n^2).

var paragraphs = document.getElementsByTagName('p');
for (var i = 0; i < paragraphs.length; i++) {
  doSomething(paragraphs[i]);
}

It is better to do this instead:

var paragraphs = document.getElementsByTagName('p');
for (var i = 0, paragraph; paragraph = paragraphs[i]; i++) {
  doSomething(paragraph);
}

This works well for all collections and arrays as long as the array does not contain things that are treated as boolean false.

This pattern is used quite frequently in blockly-core. Unfortunately, this pattern violates JSHint W084, AKA boss, which protects against assignments going in unexpected places. Most commonly, if (x = 'foo') rather than if (x == 'foo').

Note that JSHint supports the following:

You can silence this error on a per-use basis by surrounding the assignment with parenthesis, such as:

for (var i = 0, paragraph; (paragraph = paragraphs[i]); i++) {}

For consistency, we should do one of the following:

• modify our JSHint config to ignore W084 and expose ourselves to the possibility of that error.
• update our style guide to reflect that we do not allow assignment in the conditional and begin the process of updating blockly-core to reflect this. The commit below gives an example of what that would look like for a single file.
• update our style guide to reflect that we follow Google's style, but with the added requirement that you wrap your assignments in parenthesis, and begin the process of updating blockly-core to appropriately wrap all instances of this pattern in parenthesis.

Thoughts?