Rule proposal: `prefer-else-if`

Fail

if (foo === '1') {
  // ...
}
if (foo === '2') {
  // ...
}

Pass

function a() {
  if (foo === '1') {
    // ...
    return;
  }

  if (foo === '2') {
    // ...
    return;
  }
}

if (foo === '1') {
  // ...
} else if (foo === '2') {
  // ...
}

Original proposal for `prefer-switch`

if (foo === '1') {
  ...
}
if (foo === '2') {
  ...
}
if (foo === '3') {
  ...
}
if (foo === '4') {
  ...
}

To fix this case, we can simply only check if foo is declared as const.

This come from my work, but real case is more complicated, it’s like

if (foo.bar === '1') {
  ...
}
if (foo.bar === '2') {
  ...
}
if (foo.bar === '3') {
  ...
}
if (foo.bar === '4') {
  ...
}

To fix this case, we need to check foo.bar and foo didn’t get changed in each if block, I feel this is hard (even impossible) to do.

[fisker]

I changed my mind about this, I think this should be a separate rule, maybe called prefer-else-if, prefer-switch has a limitation on minimumCases,

if (foo === '1') {
  ...
}
if (foo === '2') {
  ...
}

There are only two cases, we should not ask user to use switch, but

if (foo === '1') {
  ...
} else if (foo === '2') {
  ...
}

should still be preferred, because the old one always compares foo twice, event value of foo is 1.

After we fix it to else-if, if there are many cases, prefer-switch will report.

[sindresorhus]

Sounds good.