Rule proposal: disallow chained variable assignment

When the rules will warn. Include a description as well as sample code.

Chained variable assignment creates implicit global variables. Although these should be caught by the no-undef rule, people may want to prevent chained variable assignment for readability purposes.

// Bad (this fails no-undef)
var a = b = c = 1;

// Bad (this passes no-undef)
var b;
var c; 
var a = b = c = 1;

// Good
var a = 1;
var b = a;
var c = a;

Whether the rule prevents an error or is stylistic.

This rule prevents an error if the no-undef rule is not enabled, otherwise it is stylistic and similar to the one-var rule.

Why the rule should be in the core instead of creating a custom rule.

This is a basic JavaScript syntax rule that is generally applicable to anyone writing JavaScript.

Are you willing to create the rule yourself?

Sure.

[platinumazure]

@ljharb What you’re saying makes sense and is basically what I had been thinking in my head earlier. I just wanted to give the ESLint team a chance to jump in and let me know if I had missed something.

I’ll probably champion this, once we figure out how exactly we want to do it.

[ljharb]

I think it makes the most sense as a new rule. one-var might work, but even when you don’t want to require or disallow multiple var statements, you may want to govern this pattern. no-undef also might work, but this is not saying “don’t use undefined/global vars”, it’s saying “don’t accidentally create some global vars with one local”.