Workshop › Tower Defense › Step 15 of 42

Where can you build?

What we are doing

A tower can only go on grass, inside the map, on a free tile, and only if you can pay for it. canBuild answers all of that in one place, and build refuses when the answer is no.

```js
function canBuild(col, row) {
  const inside = col >= 0 && col < COLS && row >= 0 && row < ROWS
  const taken = towers.some((t) => t.col === col && t.row === row)
  return inside && !road.has(key(col, row)) && !taken && gold >= TOWERS[selected].cost
}

function build(col, row) {
  if (!canBuild(col, row)) return
```

What it means

inside: the column and the row are both within the map.
taken: some tower already stands there (some asks "is there at least one?").
The answer is true only if all four are right: && means "and", ! means "not".

Your turn

Above function build(col, row) { write canBuild.
Write the if as the first line of build.

Skills: Game states, Collision detection

← Click to build Show the gold →

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