为什么总是返回true？

Question

Beginner question, sorry if it's not the right place to ask Trying to learn how logic works in JS , can't figure this out

if (firstSymbol === "A" || "a" || "D" || "d") {
    if (secondSymbol === "z") {
        alert("It does!"); 
        break;
    }
}

I expect it to say "It does!" and break in case if the firstSymbol is A, a, D or d AND the secondSymbol is z , but for some reason it says "It does!" and breaks regardless of what the firstSymbol is and only checks if the secondSymbol is z.

Answer 1

Because you're checking whether "a" is true - it is always true:

 console.log(!!"a"); 

You should be using includes and AND && in this case:

 const firstSymbol = "D"; const secondSymbol = "z"; if (["A", "a", "D", "d"].includes(firstSymbol) && secondSymbol == "z") { console.log("It does!"); } 

Answer 2

 function matchSecondSymbol(firstSymbol, secondSymbol) { // By making FirstSymbol Uppercase, we can remove the other two conditions firstSymbol = firstSymbol.toUpperCase(); if (['A', 'D'].includes(firstSymbol) && secondSymbol === "z") { console.log('it Does'); } else { console.log('it does not'); } } matchSecondSymbol('a', 'z'); matchSecondSymbol('z', 'z'); matchSecondSymbol('a', 'y'); 

Answer 3

In Javascript there's something called truthy and falsy values. In summary, is how a value evaluates in a Boolean ( true or false ) context.

All values are truthy unless they are defined as falsy (ie, except for false, 0, "", null, undefined, and NaN).

In your code, when you wrote:

if (firstSymbol === "A" || "a" || "D" || "d")

You are checking 4 boolean conditions:

1. firstSymbol === "A" - The result will depend on firstSymbol
2. "a" - Will always evaluate to true
3. "D" - Will always evaluate to true
4. "d" - Will always evaluate to true

So, since conditions 2, 3 and 4 will always be true , your code will always enter the if statement. If even a single one of these would be true the behaviour would be the same.

You can rewrite it in some ways:

if (firstSymbol === "A" || firstSymbol === "a" || firstSymbol === "D" || firstSymbol === "d")

or

if (["A", "a", "D", "d"].indexOf(firstSymbol) > -1)

or

if (["A", "D"].indexOf(firstSymbol.toUpperCase()) > -1)

Answer 4

Just to add something to the boilerplate, these are some examples to accomplish what you're trying to do, with tons of possibilities.

Tests covered:

• using array.includes
• using array.indexOf
• using array.find
• using array.some

WHY didn't your code work?

It didn't work because javascript evaluates the following expression: "A" || "a" || "D" || "d" "A" || "a" || "D" || "d" "A" || "a" || "D" || "d" to "A" , because "A" is truthy . If you need to compare with multiple values, either use an array, either write the condition for each of them: firstSymbol === "A" || firstSymbol === "D" ||... firstSymbol === "A" || firstSymbol === "D" ||...

Examples mentioned above:

 /* Original code */ /* if (firstSymbol === "A" || "a" || "D" || "d") {if (secondSymbol === "z") alert("It does!"); break;} */ let firstSymbol = "d", secondSymbol = "z"; // Using .includes if (["A","a","D","d"].includes(firstSymbol) && secondSymbol === "z") console.log('it does, with .includes'); // Cleverer .includes due to the nature of the input. if (["a","d"].includes(firstSymbol.toLowerCase()) && secondSymbol === "z") console.log('it does, with cleverer .includes'); // Using .indexOf if (["A","a","D","d"].indexOf(firstSymbol) > -1 && secondSymbol === "z") console.log('it does, with .indexOf'); // Using .find if (["A","a","D","d"].find(i => i === firstSymbol) && secondSymbol === "z") console.log('it does, with .find'); // Using. some if (["A","a","D","d"].some(i => i === firstSymbol) && secondSymbol === "z") console.log('it does, with .some'); 

Answer 5

Order of Precedence decides

In each programming language, symbols are processed in order of their precedence order.

In Short:

AS the others already explained, your assignment

firstSymbol === "A" || "a" || "D" || "d"

would be processed as

(firstSymbol === "A") || ("a") || ("D") || ("d")

Link to how logical operators are processed: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence