lodash_startsWith.js

function startsWith(string, target, position) {
  debugger;
  const { length } = string
  position = position == null ? 0 : position
  if (position < 0) {
    position = 0
  }
  else if (position > length) {
    position = length
  }
  target = `${target}`
  return string.slice(position, position + target.length) == target
}

export default startsWith

my_startsWith.js

function startsWith(string, target, position) {
  const { length } = string
  position = position || 0
  target = `${target}`
  return string.slice(position, position + target.length) == target
}

export default startsWith

startsWith('efg', 'h', 4) should return false
startsWith('efg', 'd', -1) should return false
startsWith('efg', '', 4) should return true
startsWith('efg', '', -1) should return true

Both versions do this, buy my version removes redundant code and save space.

position = position == null ? 0 : position

can be expressed as

position = position || 0

because if position is null or undefined or false, 0 is used else position

startsWith('abc','a') should return true

if (position < 0) {
position = 0
}
else if (position > length) {
position = length
}

can be dropped because

target = 'a'
'abc'.slice(-1,-1+target.length) == ''
'abc'.slice(4,4+target.length) == ''

so the bounds check is redundant.