This is the bash script I use to install GHCJS (8.6) on Linux

install-ghcjs.sh

#!/usr/bin/env bash
# forked from Moritz Angerman https://github.com/angerman/haskell-to-web/blob/master/ghcjs-build-notes.org

set -e pipefail

cabal new-install happy --overwrite-policy=always
cabal new-install alex --overwrite-policy=always
cabal new-install hscolour --overwrite-policy=always
cabal new-install hsc2hs --overwrite-policy=always

rm -rf "$HOME/.ghcjs"
mkdir -p "$HOME/.ghcjs"
cd "$HOME/.ghcjs"
git clone --branch ghc-8.6 https://github.com/ghcjs/ghcjs.git
cd ghcjs
git submodule update --init --recursive
./utils/makePackages.sh

cabal new-build

mkdir bin
(cd bin && for a in ghcjs ghcjs-pkg ghcjs-run haddock-ghcjs hsc2hs-ghcjs ghcjs-boot; do ln -s ../utils/dist-newstyle-wrapper.sh $a; done)
export PATH=$(pwd)/bin:$PATH
ghcjs-boot -s data/boot.tar --no-haddock