如何在 JavaScript/TypeScript 中动态读取/写入局部变量？

Question

I am pretty new to angular and was wondering how I can access a LOCAL variable dynamically. Here is the scenario I am facing:

I am declaring 7 objects within an IF statement:

if (...) {
   const day1 = { date: null, total: 0 };
   const day2 = { date: null, total: 0 };
   const day3 = { date: null, total: 0 };
   const day4 = { date: null, total: 0 };
   const day5 = { date: null, total: 0 };
   const day6 = { date: null, total: 0 };
   const day7 = { date: null, total: 0 };
}

Later within that IF statement, I need to assign a value to the "total" key depending on calculations also done within that IF statement.

Now, instead of doing something like this later within the IF statement:

day1.date = ... ;
day2.date = ... ;
day3.date = ... ;
day4.date = ... ;
day5.date = ... ;
day6.date = ... ;
day7.date = ... ;

I would like to do something like this to keep my code neat and efficient (obviously, this does not work):

for (let i = 1; i <= 7; i++) {
  ['day' + i].date = ... ;
}

It would have worked if the 7 objects were declared globally and then using this['day' + i].date . But the objects being declared locally in the IF statement, I can't use this syntax.

My questions are the following:

1. Is there something similar for locally declared variables?
2. Even better, can I declare these 7 local objects dynamically in a for loop?

Thank you for your support.

Answer 1

Best solution: Use an array instead:

Change this:

if (...) {
    const day1 = { date: null, total: 0 };
    const day2 = { date: null, total: 0 };
    const day3 = { date: null, total: 0 };
    const day4 = { date: null, total: 0 };
    const day5 = { date: null, total: 0 };
    const day6 = { date: null, total: 0 };
    const day7 = { date: null, total: 0 };

   // ...

    day1.date = ... ;
    day2.date = ... ;
    day3.date = ... ;
    day4.date = ... ;
    day5.date = ... ;
    day6.date = ... ;
    day7.date = ... ;
}

...to this:

// Declared in module scope:
type Day = { date: Date | null, total: number };

// Inside your function:
if( ... ) {
    
    const days: Day[] = [
        { date: null, total: 0 },
        { date: null, total: 0 },
        { date: null, total: 0 },
        { date: null, total: 0 },
        { date: null, total: 0 },
        { date: null, total: 0 },
        { date: null, total: 0 }
    ];
    
    //

    for( let i = 0; i < 7; i++ ) {
      days[i].date = ...
    }
}

You can also initialize days in a loop too:

const days: Day[] = [];
for( let i = 0; i < 7; i++ )
{
    days[i] = { date: null, total: 0 };
}

And you can have named references to elements in the array if you want to keep on using day1 , day2 , etc (but remember these are references to the object values in the array rather than being aliases to array indexes):

const day1 = days[0];
const day2 = days[1];

...though try not to let your 1-based day numbering be confused with JavaScript/TypeScript's 0-based array indexing.

Worst solution: Using eval :

You can manipulate locals using eval() , but you shouldn't use eval() , ever . But you can do this:

if (...) {
    const day1 = { date: null, total: 0 };
    const day2 = { date: null, total: 0 };
    const day3 = { date: null, total: 0 };
    const day4 = { date: null, total: 0 };
    const day5 = { date: null, total: 0 };
    const day6 = { date: null, total: 0 };
    const day7 = { date: null, total: 0 };

    for (let i = 1; i <= 7; i++) {
        const expr = 'day' + i + ' = ...;';
        console.log( expr );
        eval( expr );
    }
}