Undefined/Can't Run In Coding Blocks

I code random/fun things in the coding blocks and I came across a small problem with the blocks/javascript 

I was coding in blocks when unusually I could not run my code no errors showed anywhere on the blocks so I went into javascript it showed my non-array variables were 
let myvarname: number = []

so I changed the [] to 0 and it worked fine so I switched back to blocks and the error came up again saying I can't run the code I switch back and again its []