JS-Object.keys（）中的三元语句

Question

When I put a ternary statement or an if statement inside the Object.keys() like bellow:

 Object.keys(date).forEach((index) => {
        dates[date[index].id] !== undefined ?
          dates[date[index].id] =
          [...dates[dates[date[index].id], dates[date[index].name]
          : null;
      });

I get this error from the Linter:

Expected an assignment or function call and instead saw an expression. (no-unused-expressions)

When I use a regular if statement, I get this error,

Parsing error: Unexpected token (Fatal)

Why?

Answer 1

You've duplicated dates in and got some missing brackets in the assignment:

dates[date[index].id] = [...dates[dates[date[index].id], dates[date[index].name]

Which should be:

dates[date[index].id] = [..dates[date[index].id], dates[date[index].name]]

Demo:

Object.keys(date).forEach((index) => dates[date[index].id] !== undefined ?
        dates[date[index].id] = [...dates[date[index].id], dates[date[index].name]] : null);

Answer 2

You could use a regular if condition, without a ternary expression, which the linter does not like.

Object.keys(date).forEach((index) => {
    if (dates[date[index].id] !== undefined) {
        dates[date[index].id] = [
            ...dates[dates[date[index].id]],
            dates[date[index].name]
        ];
    }
});

Basically the tenary is used with an assignment outside of the operator, like

x = a > 4 ? 7: 8;

whereas your assignment takes place inside of the operator.

Answer 3

You just have a few syntax errors in there, probably stemming from the overcomplicated nesting of property accesses. Keep it simple:

for (const index in date) {
    const id = date[index].id;
    if (dates[id] !== undefined) {
        dates[id].push(date[index].name);
        // or if you insist:
        // dates[id] = [...dates[id], date[index].name];
    }
}