如何使用switch而不是多个if语句？

Question

I was wondering if it is possible to rewrite multiple if statements into a switch .

The problem is that a switch runs:

1. all code after a case passes a check. Which is why the case statement runs all code after the first case.

    let arr = [1, 3]; if( arr.includes(1) === true ) { console.log('if 1'); } if( arr.includes(2) === true) { console.log('if 2'); } if( arr.includes(3) === true) { console.log('if 3'); } switch( true ){ case arr.includes(1): console.log('switch 1'); case arr.includes(2): console.log('switch 2'); case arr.includes(3): console.log('switch 3'); } 

   2. if a switch has breaks in every case, it runs a single case, that passes the test.

 let arr = [1, 3]; if( arr.includes(1) === true ) { console.log('if 1'); } if( arr.includes(2) === true) { console.log('if 2'); } if( arr.includes(3) === true) { console.log('if 3'); } switch( true ){ case arr.includes(1): console.log('switch 1'); break; case arr.includes(2): console.log('switch 2'); break; case arr.includes(3): console.log('switch 3'); break; } 

So the question is: How can I rewrite multiple if statements into a single switch statement?

If I can't: Is there another more elegant syntax than the multiple if statements, that makes it apparent that I am making similar comparisons?

Answer 1

How can I rewrite multiple if statements into a single switch statement?

You can't, reasonably, if you want multiple cases to match. switch can replace if / else , but not a series of independent if s where more than one can match.

Is there another more elegant syntax than the multiple if statements, that makes it apparent that I am making similar comparisons?

The answer here will tend to be specific to the code you're writing. A couple of options for you:

Parameterize into a function

Whenever you have code where you're doing the same thing over and over again, parameterize it and put it in a function, then call the function repeatedly with the parameters.

function doTheThing(value) {
  if (arr.includes(value)) {
    console.log('xyz ' + value);
  }
}

For instance, in your example:

 function doTheThing(value) { if (arr.includes(value)) { console.log('xyz ' + value); } } let arr = [1, 3]; doTheThing(1); doTheThing(2); doTheThing(3); 

or

 let arr = [1, 3]; [1, 2, 3].forEach(value => { if (arr.includes(value)) { console.log("xyz " + value); } }); 

or combining those:

 function doTheThing(value) { if (arr.includes(value)) { console.log('xyz ' + value); } } let arr = [1, 3]; [1, 2, 3].forEach(doTheThing); 

Lookup table with actions as functions

If you're doing different things, one common practice is to have a lookup table of value-to-action, eg:

 const actionsByValue = { 1() { console.log("This is the thing for #1"); }, 2() { console.log("This is something else for #2"); }, 3() { console.log("Different logic again for #3"); } }; const nop = () => { }; let arr = [1, 3]; arr.forEach(value => { (actionsByValue[value] || nop)(value); }); 

That 1() { } notation may look odd because you don't see method notation with properties with numeric names very often, but it's perfectly valid. In old environments that don't support method notation:

const actionsByValue = {
  1: function() {
    console.log("This is the thing for #1");
  },
  2: function() {
    console.log("This is something else for #2");
  },
  3: function() {
    console.log("Different logic again for #3");
  }
};

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Side note: You never need === true with Array#includes . It always returns a boolean.