Example.js

const output = {
  _: ["a", "b"],
  foo: "bar",
  baz: true
};

const sgray = ({ _, ...input }) =>
  [
    ..._,
    ...Object.keys(input).reduce((acc, key) => {
      const value = input[key];
      return [...acc, value === true ? `--${key}` : `--${key}=${value}`];
    }, [])
  ].join(" ");

console.log(sgray(output));