By Using Javacript "Undefined" (Variable) we can Free system memory?

By Using Javacript "Undefined" (Variable) we can Free system memory?

I am working for an internet speed test application , The site is
calculating internet speed using javascript . When downloading larger
files 250Mb+ The browser get crashed . (Chrome , opera , Firefox ) . It is
saving all test files to dom . I saw that the memory usage of the browser
goes up crazy while testing 100Mbps + speed connection .
My question is there is any way to limit the memory usage of browsers , Or
there is any limit on using browser DOM ? After using a file Make it
"Undifined" actually delete the item from system memory ?
I say this browser crash issue after downloading 250Mb + data (next file
250 Mb so 512mb saved in dom )