Use const/let instead of var

[domoritz]

Merge after #210

@curran

[Fil]

Using const and let adds almost 200 bytes (1%) to the compressed script.

16142 d3-scale.min.js (var)
16328 d3-scale.min.js (const-let)

I've tried collapsing vars (let x,y,z…) but terser is already doing it, and returns the same file.

For the sake of science I tried to replace all const by let, and the resulting size was 16156 bytes (+0.1%). [Of course we would not do that since it defeats the purpose of using these precise const/let declarations.]

(EDIT: when comparing the gzip'ed minified, we're talking of a difference of 20 bytes.)

[curran]

LGTM.

I love how it makes the code more clear in the sense that you know what can be expected to change with reassignment vs. not.

[curran]

This change is great. Merging this PR would set a good precedent and open the door for future PRs that upgrade var to let and const across all of D3.

[domoritz]

Do you still plan to merge this pull request now that d3 6 is out?

human error! sorry