Migrate from vars to ES6 syntax

[puzpuzpuz]

• Migrates from vars to ES6 syntax, sot that there are no more vars in the code base
• Also fixes config typo in check.js

[mcollina]

Can you include the results for the benchmark on Node 10, 12 and 14? Those changes usually decrease performance.

[puzpuzpuz]

Can you include the results for the benchmark on Node 10, 12 and 14? Those changes usually decrease performance.

Sure. I did 10 runs of the benchSonic*1000 benchmark on this PR and the latest master. Didn't bother with benchmark/compare.R core script, but I can use it if you'd like to have a further analysis on the results.

Summary:
I didn't notice signs of noticeable performance regression. Interestingly, that Node.js 12 is significantly faster than both 10 and 14. To me, it looks like a regression in v12->v14.

This PR:

# Node v10.22.1
benchSonic*1000: 2672.872ms
benchSonic*1000: 2694.200ms
benchSonic*1000: 2692.539ms
benchSonic*1000: 2687.123ms
benchSonic*1000: 2681.746ms
benchSonic*1000: 2723.968ms
benchSonic*1000: 2695.435ms
benchSonic*1000: 2712.366ms
benchSonic*1000: 2694.966ms
benchSonic*1000: 2696.207ms

# Node v12.18.4
benchSonic*1000: 1874.604ms
benchSonic*1000: 1911.222ms
benchSonic*1000: 1933.655ms
benchSonic*1000: 1940.520ms
benchSonic*1000: 1932.524ms
benchSonic*1000: 1894.583ms
benchSonic*1000: 1905.953ms
benchSonic*1000: 1920.437ms
benchSonic*1000: 1929.686ms
benchSonic*1000: 1906.603ms

# Node v14.12.0
benchSonic*1000: 2.400s
benchSonic*1000: 2.416s
benchSonic*1000: 2.396s
benchSonic*1000: 2.395s
benchSonic*1000: 2.382s
benchSonic*1000: 2.377s
benchSonic*1000: 2.397s
benchSonic*1000: 2.394s
benchSonic*1000: 2.372s
benchSonic*1000: 2.394s

Latest master:

# Node v10.22.1
benchSonic*1000: 2687.569ms
benchSonic*1000: 2719.671ms
benchSonic*1000: 2704.618ms
benchSonic*1000: 2711.833ms
benchSonic*1000: 2678.593ms
benchSonic*1000: 2704.299ms
benchSonic*1000: 2693.906ms
benchSonic*1000: 2702.797ms
benchSonic*1000: 2701.660ms
benchSonic*1000: 2703.075ms

# Node v12.18.4
benchSonic*1000: 1909.902ms
benchSonic*1000: 1887.095ms
benchSonic*1000: 1892.965ms
benchSonic*1000: 1850.140ms
benchSonic*1000: 1898.076ms
benchSonic*1000: 1898.124ms
benchSonic*1000: 1931.312ms
benchSonic*1000: 1902.266ms
benchSonic*1000: 1885.535ms
benchSonic*1000: 1856.379ms

# Node v14.12.0
benchSonic*1000: 2.365s
benchSonic*1000: 2.412s
benchSonic*1000: 2.408s
benchSonic*1000: 2.409s
benchSonic*1000: 2.416s
benchSonic*1000: 2.401s
benchSonic*1000: 2.420s
benchSonic*1000: 2.411s
benchSonic*1000: 2.392s
benchSonic*1000: 2.402s

[mcollina]

lgtm