Fallowing ESLint 规则与 else if

Question

I have this code else/if code

       if (_.has($scope.item, 'comment_id')) {
          track(one);
        } else if (_.has($scope.item, 'post_id')) {
          track(two);
        } else {
          if ($scope.followers) {
            track(three);
          } else {
            track(four);
          }
        }

but ESlint want's me to turn it into this

        if (_.has($scope.item, 'comment_id')) {
          track(one);
        } else if (_.has($scope.item, 'post_id')) {
          track(two);
        } else if ($scope.followers) {
          track(three);
        } else {
          track(four);
        }

Are they the same thing?

Answer 1

Yes, they are equivalent. ESLint is smart enough to detect that and hence the suggestion. The reason is that you only really have four alternatives in the if/else construct - when the code doesn't match any of the first two conditions, it will always execute the outer

else {
  if ($scope.followers) {
    track(three);
  } else {
    track(four);
  }
}

And that will only ever result in one of two things happening. Extracting the if ($scope.followers) as an else if follows the exact same logic path.

This can be demonstrated very easily by abstracting away the conditions and generating a truth table, then checking the results. It can be done on paper but since you already have the code, it's also easy to make it as code:

 function nested(a, b, c) { if (a) { return "track(one)"; } else if (b) { return "track(two)"; } else { if (c) { return "track(three)"; } else { return "track(four)"; } } } function chained(a, b, c) { if (a) { return "track(one)"; } else if (b) { return "track(two)"; } else if (c) { return "track(three)"; } else { return "track(four)"; } } const truthTable = [ [false, false, false], [false, false, true ], [false, true , false], [false, true , true ], [true , false, false], [true , false, true ], [true , true , false], [true , true , true ], ]; for(const [a, b, c] of truthTable) { const nestedResult = nested(a, b, c); console.log(`called nested() with a=${a} b=${b} c=${c} result: ${nestedResult}`); const chainedResult = chained(a, b, c); console.log(`called nested() with a=${a} b=${b} c=${c} result: ${chainedResult}`); console.log(`matching results?: ${nestedResult === chainedResult}`); console.log(`------------------------`); }

Alternatively, you can generate an actual truth table to visualise the results:

 function nested(a, b, c) { if (a) { return "track(one)"; } else if (b) { return "track(two)"; } else { if (c) { return "track(three)"; } else { return "track(four)"; } } } function chained(a, b, c) { if (a) { return "track(one)"; } else if (b) { return "track(two)"; } else if (c) { return "track(three)"; } else { return "track(four)"; } } const truthTable = [ [false, false, false], [false, false, true ], [false, true , false], [false, true , true ], [true , false, false], [true , false, true ], [true , true , false], [true , true , true ], ]; const enrich = truthTable .map(row => row.concat(nested(...row), chained(...row))) //add the results of the two calls .map(row => row.concat(row[row.length-1] === row[row.length-2])) //compare the last two const table = document.querySelector("table"); for (const rowData of enrich) { const newRow = table.insertRow(-1); for (const rowValue of rowData) { const cell = newRow.insertCell(-1); cell.textContent = rowValue; } }

 table { border-collapse: collapse; } table, th, td { border: 1px solid black; }

 <table> <tr> <th>a</th> <th>b</th> <th>c</th> <th>nested result</th> <th>chained result</th> <th>results equal</th> </tr> </table>